If I understand correctly you applied under section 6(1) (in your own right) rather than as spouse of a British citizen.
Reading the official guidance at page 11
https://assets.publishing.service.gov.u ... t_2023.pdf
Requirement to have been in the UK on
the first day of the qualifying period
If you are applying under section 6(1), you must have been in the UK exactly 5 years
before your application was received. For example, if your application is received on
05/01/2022 you should have been physically present in the UK on 06/01/2017.
If you are applying under section 6(2), as the spouse or civil partner of a British citizen,
you must have been in the UK exactly 3 years before your application was received. For
example, if your application is received on 05/05/2021, you should have been physically
present in the UK on 06/05/2018.
If you cannot meet this requirement, your application is likely to be refused. If you believe
there were exceptional circumstances which prevented you from being in the UK on the
first day of the qualifying period, please provide details when you apply.
We recognise that occasionally someone might apply without realising that they were not
in the UK at the start of the qualifying period. If you did not meet the requirement when you
applied, we may see if there is another, later date we can use to allow you to meet the
requirement and will consider your application on, or after, that date.
However, please be aware the requirements remain unchanged, and this does not
guarantee that the application will be successful if an alternative date cannot be found. It
may also result in delays to your application until you can meet the statutory requirements.
You must pay the full fee that is in force on any date we use.
You are strongly recommended to make sure you meet the requirement before
applying.
Your application is likely to be considered from the 31 Jan/1 Feb 2019. You could write to UKVI explaining you didn't realise you were absent on the first day of your qualifying period.
All advice comes from personal research and experience and should not be regarded as professional opinion.