I think it's more likely that a software update made the CAR more sensitive to some sort of wiring issue (it's looking at something more carefully/has tighter parameters on what it will tolerate), or some other issue that varies between cars.
I can't think of another explanation for this issue and a similar issue in the other thread (though they seem to be somewhat different).
The only fly in the ointment is that there is one report in the other thread that is not coincident with a software update (the problem allegedly first occurred BEFORE a software update).
It would be interesting to see pictures from people of their grounding situation for their charging setups (at the outlet, at the subpanel if applicable, and at the main panel). For setups which work, and setups that don't.
It's not the UMC or WC checking the ground continuity (they already do that) that is causing problems, it seems to be car-related...
It could also be the software update is finding issues (variation) between CARS + WIRING that is causing problems (it's entirely possible there are small variations in the charger circuitry in the car and the software may not be compatible/properly tested on all such variation). The way to rule this out is to have someone with an "ok" Model 3 come over to one of the "problem houses" and try the charging setup. It's presumably not just the variability in the car itself ALONE that is the issue, otherwise the car would not charge elsewhere (in general they seem to work fine elsewhere).
This is the other thread I am referring to:
Charging Issue Level 2 at Home