The Pump House above has all the right info for you.

My Fish Tales has a metal piece of spring steel to spring the opto interrupt back after the flipper button is released, but I don't know if that's an aftermarket fix, or if the machine was sold with that.

Does your flipper button opto use a plastic flipper interrupt with no spring steel to push it back into position when the flipper button is released?

One thing I would mention is that the plastic flipper opto interrupts on the flipper buttons had a design flaw in the original plastic.

According to a service notice in StarTech journal back in the day, the plastic part would get too flexible, and not 'quite' activate the flipper, but actually be activating for a fraction of a second and cause the flipper coils to get warm... which leads us to what The Pump House says above.

I personally have had the original plastic opto interrupt seem 'spongy'. You would push the flipper button, and if you were very familiar with how fast the opto interrupt should snap back, you could notice that it was slow to bend back to a fully closed position.

I personally never had a pinball with warm coils due to this, but I definitely replaced the original plastic flipper button opto interrupt plastic with the newly made ones that use a different material, and that fixed my flipper problems.

Another something to check.