[Fern]

Explosion will not give you momentum if it hits an enemy weak to fire, or reduce it if you hit an enemy that resists fire.
 
Tested weakness against Goblins.
Tested resist against Terra Flama enemies (Salamandra, Hot Lips, etc).
 
This was not the case before, so I'm assuming it's not intended.

[Neus]

The damage likely takes place before Momentum is set for the round. I don't know if I'd call it a bug.