I think you should complain to the university, and make sure they understand the devastating collateral damage to you of their choice to report this matter, as someone who was outside the UK when they were suddenly told their visa was curtailed for "ending early".
Reporting to the Home Office that someone has completed their course early is normal and is required (see below), but usually it is only done when someone finishes significantly early, like a semester or a year early, and usually when it happens through their own choice to withdraw with a lower qualification, complete a PhD way ahead of schedule, etc. That is a totally different matter to your course ending exactly as planned, just that it happens to be a very short time before the end date the university quoted on your original CAS three years ago. As you say, as far as you were concerned you were completing as had always been planned.
There is more. Even when someone is legitimately reported for completing early and has their visa curtailed, they will still have the +4 months wrap-up period. The fact that your visa was curtailed to 60 days, not the normal +4 month wrap up time, suggests to me that you may have been reported wrongly as having withdrawn from the course, not completing early. This would not have helped someone like you who was outside the UK when the curtailment happened anyway, but as I say no report for early completion was even necessary.
The university may well say that there were following the Home Office's rules by reporting students who compete early. However, this is bogus. You can see from the Home Office's own guidance to universities on this matter (below) that there is no such rule, no need to report this matter, nor any realistic cause for concern about their CAS end date -- if that was the university's genuine concern. Of course, the "incorrect" CAS end date is nothing to do with you or any change to your course anyway.
This from the the Sponsor Guidance, document 2 "Sponsorship duties", paragraph 3.12. Note the text in red.
3.12 We recognise that sponsors’ academic schedules can change, and that it may therefore be difficult for you to pinpoint the end date of a course precisely when assigning a CAS. If changes to the academic timetable means that the student’s course finishes one or two weeks earlier or later 13 than the course end date given on the CAS, this will not be considered as non-compliance with sponsorship duties. However, if there is evidence that a sponsor repeatedly gives course end dates that are significantly later than students are expected to complete their studies, that will be considered to be a breach of sponsorship duties. Further, it may be considered best practice to continue to make SMS notifications on change of student circumstances where course end dates have changed within the two week range, as the course end date informs when a Student is permitted to switch into work routes.
https://www.gov.uk/government/publicati ... r-guidance
Note that the last sentence suggests only that "it may be considered good practice" to report all early completions, not that it is a requirement to do so. It seems to me that the university's approach is fully focussed on protecting themselves against an unlikely accusation of using a wrong end date on your CAS, with no sense of proportion or reasonableness.
When choosing to report someone like you as if they have completed early, the university is neglecting the equally and in this situation more important "good practice" of showing a duty of care to students and being mindful of the consequences of their actions. In this case choosing to report you for "completing early", which you didn't even know you were doing, let alone choosing to do, you have been blocked from returning to the UK on your Student visa and from applying for the Graduate visa. These are devastating consequences of a thoughtless university policy -- or worse, someone's personal decision -- and the university needs to know the damage they have caused.
I assume that the international student advisers at Greenwich work in students' interest, not only in the university's interest, so you might want to raise this with them. If you make a formal complaint to the University, I would advise getting support from the Students Union and maybe getting a sense of how many people have been affected. As well as people like yourself who were outside the UK when this decision was made to make the report (hopefully not very many), others may be affected. For example, someone planning to apply for the Graduate visa but whose visa will now expire before they get their results, and so cannot apply.
You have no reason to follow or heed the advice of an anonymous contributor to this forum, and of course my response is based on me taking everything you have said at face value. It seems believable to me, if also shocking. Your university may have a different view of what has happened. Hopefully you can see that I feel strongly about this and that I seem to know what I am talking about! I do work in the higher education sector, and I think bad practice by sponsors should be called out.
I wish you well, and I hope you pursue this matter because you seem to have been done a real disservice. I disagree with the comment that says you took a risk by travelling when your course had already finished. You had no reasonable expectation that your visa would be curtailed, for all the reasons described above, and it is absolutely normal practice for students to travel and re-enter the UK after studies. It is provided for in the Student route caseworker guidance:
Students are able to travel outside of, and re-enter, the UK whilst they hold valid permission as a Student, including in the period after they have completed their course and still hold permission under the route.
https://www.gov.uk/government/publicati ... dent-route > page 89
I do not give immigration advice. I refer you to Immigration Rules, guidance, other online content and to your sponsor.
