I don't know how much this is related to chessx but I've found a situation where the tablebase is wrong, here is the FEN:
K2B4/4P3/k7/3n4/8/8/8/8 w - - 1 4
[+] Tablebase: 4.e8=Q - White wins in 21 moves
[+] Tablebase: 4...Nc7+ - Draw
The correct move is 4.e8=R
I've made an youtube video:
I don't know what tablebase chessx uses but somebody should recheck it !