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Nationals of
the UK residing in Spain before the date of withdrawal shall be obliged to
request ... documentation corresponding to the authorisation of temporary
residence ...
Applications
must be submitted before the end of the 21 month period commencing after the
date of withdrawal.
The procedure
for obtaining this authorisation is established according to whether the
national of the UK is the holder of a temporary registration certificate
obtained before the withdrawal date, or lacks this.
However, the law does not define how to prove you were resident before the Brexit date. In addition, it will be up to the national police stations and foreigners offices as to how they interpret the law. For example, I could make a good case for stating that if you had only been here for less than 90 days before the exit date, then you were here on a stay and not as a resident.
On top of that, the law was issued by the previous government, and there's no guarantee that the present government won't amend or rescind it which, given the intransigence displayed by Boris, I consider quite likely.
Given also that non-EU citizens have to prove not only health cover, but far more significant financial resources than do EU citizens, then I think it is sensible to move here and get a residency certificate before the deadline.