has it been breezy? the young foliage loses water faster than when they are older/thicker and tougher.

if they are both showing the limp look that's what I'd suspect.

one out of my 14 did this last year, I moved it as it looked asif it was drying out, gets plenty of water but mines doing it again too this year :)

It's been breezy, but not excessively, and one is in a very sheltered spot. They're both new trees last year, so still very young. I have another Acer, different variety and that's in the same area and doing fine :(