I’ll take an ‘eyes wide shut’ moment to figure this one out. 
edit: For below, the whole set of tags on that node
access=yes
amenity=charging_station
capacity=2
check_date=2022-08
fee=yes
name=ABB
operator:wikidata=Q52825
operator=ABB
payment:app=yes
I can’t get this to repro by adding a node with that tagging. Curious what triggers it on your machine.
Reproducible here, with any month before -09 .The months 9,10,11,12 don’t invoke the quirk, or is it an undocumented feature ;P)
My suspicion is the abbreviation of the date is setting something off i.e. without a day. A full date 2022-08-31 is fine.
AND 2022-8 is fine too,
I’m not sure, but I think I remember JOSM compares with other values in the near surrounding.
So if there are much more check_date=2022-09 in the area JOSM assumes that check_date=2022-08 might be wrong / a typo.
I’ve downloaded that node and some of the surrounding area. Still can’t get the validator warning to trigger. Super odd.
Had a hunch it was related to the MapWithAI plug-in. Since it irritated anyhow with spurious validation warnings, printing it was experimental, and the building outlines not up to my snuff in general… stopped using it, I removed it, but no, the message still there. Why without leading zero in the 1-8 months it’s fine is odd. The historic date wiki sample does indicate it’s part of the abbreviated format.
Had hoped maybe a hidden unprintable char. Deleting the date value and reentering just makes it come back. so leaving it alone for now without resolution.
I doubt this warning has anything to do with dates or check_date in particular. It’s probably some logic that checks values against a list of known values (maybe from a preset or from data that was downloaded before), and if your value is not among the known values, but looks “very similar” to one that is, you get a warning.
That’s probably because with the missing zero it no longer looks “very similar” to the known value.
If it happens with updated JOSM, and without plugins and custom validator rules then Help -> Report bug would be a good thing to use here
looks like a bug to me
Clean start up, new layer, smallest possible data download, just the ABB charging POI and a few parking spots, moved the charger and validator goes off on it with same warning, so it’s very likely something in my combo of plugins and presets. Issue in park, too rare to spent more time on.
NB when typing 2022-08 with the full dataset loaded I’m getting 2022-08-31 offered for auto completion.
Sometimes JOSM reports an error that doesn’t look like an error to me. It would be nice to be able to report that as a false positive.
Hmm 
You report such cases of systematic issues with Help → report bug