So this has been happening for a while and I am not sure what is causing it, but occasionally yet consistently doing certain activities like turning in Unique/Untradeables for GC seals causes the ability on hotbar 1 to be replaced with the Meld Materia action, regardless of what ability was there, and regardless of what class I am on. Some other times it happens randomly like after talking to NPCs for quests. Locking the bar from being changed has no effect on this bug, and it doesn't happen 100% of the time, probably about 40%.