Yes it is
To make it simple, it's not about what right you have but it's about your ability to
demonstrate that right.
In the UK, employing a foreign national is a serious liability for any employer. Because if they hire someone who doesn't have the right to work, they can be fined and get serious legal troubles.
As immigration law is complex and confusing, HR people don't want to go too much into that when hiring. They just want to see a document from the Home Office certifying that the person has the right to work or similar device such as an official website where they can check that.
I know a company in London where you can't work with an EEA passport. They want to see an settlement scheme CoA or confirmation email. This is because when facing liabilities, businesses go beyond the minimal legal requirements in place. They push the boundaries as far as possible to be in the safe side.