Ephara Discussion
Semantics: We don't actually downgrade DEC if the opponent confirms the illegal draw, but rather we skip DEC entirely and (generally) call it a GRV.
This situation as it stands however is definitely a DEC-GL. They've missed their trigger from the previous upkeep and doing so now is not remotely close to being “the first time it would affect the game in a visible fashion” - namely, player A having an additional card in his hand after the prior upkeep.
The only reason we might possibly resolve this as an actual Missed Trigger infraction would be if either player noticed and called attention to the ability before drawing cards. They didn't, so the first thing that really has gone wrong that we have to step in on is the card draw itself. There's no real out to do otherwise, in my opinion.