Melee attack when trying to use ultimate ability

Sometimes when trying to use an ultimate ability, the character performs a melee attack instead (which has a completely separate keybind). At times it will eventually succeed, but often repeated attempts still fail - the one thing in common is that the situations where it fails are hectic. I have experienced this issue at least on Caustic and Mad Maggie, in both Wildcard and Team Deathmatch; however I do not know how to reproduce it. Platform is PS4.