I would have thought 1 was the only option. 2 would make no sense; can’t think of any situation where a limit would be increased and then decreased so quickly like that.
I can’t see it being anything other than this, once applied to your Flex limit your new limit is £3k but you only have until 20th December to accept the limit increase.
It’s an interesting use of time pressure isn’t it? I’m sure the community will have opinions on this.
So behind the scenes one of two things is happening:
Monzo have pre-approved you for a limit rise based on your standing at a previous date. This pre-approval ends on the 20th, however it’s possible that you’d still get exactly the same offer after the 20th if you remain in the same standing. i.e. the time period they’ve given you may not matter, but the wording makes you feel like you need to take action before the date.
Monzo may have relaxed their lending criteria slightly for a short period and pre-approved you.