Is the return spring missing?

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Take the glass off and manually move the flippers. Do they move smoothly with no resistance.

start a game without hitting the flippers. Manually move the flippers to their up position with your hand.

Does the flipper stay up and resist being pulled back down?

If the flipper stays up the hold winding in the coil is energized when it shouldn’t be which is either a short to ground or a board issue.

Another possibility is your flipper switch optos. Go to the test menu, switch levels and take a picture and post it here

I assume you checked your see if the coil stop is mushroomed & magnetized?

You say flipper drops if you turn off game or open the coin door. That likely rules out a mechanical problem. It sounds like the coil power is getting stuck on. I’m with Pump House and in my own experience this has always turned out to be a board problem. You can ship boards to be repaired for a reasonable cost. First you want to try to narrow it down if possible.

I schould check also the working of the flipper opto's

No. The switch test will show a grid of boxes. When you activate a switch the box corresponding to it will light up. Take a pic of that screen.

Remove the glass. With a game running, manually move the flipper to the up position and see if it stays there and resists being pulled back down.

This is definitely not the switch, as this is only releasing when the power is shut off(either with the power switch or the coin door kill switch). Use the manual:
https://www.ipdb.org/files/3915/Bally_1996_Scared_Stiff_Manual.pdf
If this is the left side, you are looking at Q87 and Q89. Follow the instructions in pinwiki(https://www.pinwiki.com/wiki/index.php/Williams_WPC#Coil_Locks_On) to test these transistors. You can easily replace these, or send out the relevant boards for repair.

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