Given how cache placement rates are increasing, finding a cache for the last month should be pretty easy, and the further you go though the current month the easier it gets. Covid does make things a little awkward, but luckily there are some countries that continued cache placements throughout April. So possibly to qualify for this challenge, as with qualifying for other challenges, you may have to go overseas. Once international travel is relaxed.
Project-GC's Hidden Months tool does ignore the current month and any checker requiring the current month to be included IMHO is asking a bit much. If the checker checks the current month and fails you on that then use the PGC Finds by Hidden Month chart as your qualifying proof.
Re qualifying at the time and later unqualifying, there are a number of stats that can be unwound - cache ratings can be changed, favourites removed, etc. These could all affect challenges. Except that they don't. All you need to do is provide proof that you qualified at the time.
Ignoring the current three months is pointless as by the time October comes around you would still have a hole in April.
And April 2020 should not be exempted from Jasmer!