I only started playing again for update 35 so I don't know how old this is but it's sure annoying. If you're using either E or M1 to do attacks with any weapons, and close enough to be within finisher/mercy range, there's a small chance for the game to totally ignore YOUR plans and force you to do one instead of pressing X. This happens regardless of the "Melee with Fire Weapon Input" option setting, I turned it on and off and still encountered the issue multiple times every day this week.
Best case scenario: I do a mercy attack on an eximus that I was planning to kill anyway. Whatever.
Worst case scenario: Without my involvement, the game makes me do a lich/sister mercy attempt I didn't want to do, and then moves them to another planet.
It may be related, but sometimes the 'X to land on zipline' executes by itself too.