It may be vibration that is triggering the switch. Put the game into switch edges test, then pound on the playfield with your fist.

If you've already reseated the connectors on the opto board, there may be a connection issue at one of the optos. Also possibly a problem with the opto board. I'm guessing even the pro has more than one opto board. If so, swap one with Sparky board and see if the problem moves.

The grave marker switch (optos), at the back of the drop targets, is connected to the same opto board as the electric chair optos. Is that switch working?

The opto board used for both of those switches should have two red LEDs lit when the nothing is blocking the optos. Are you seeing those two LEDs lit?

With the power off, carefully swap the four small connectors on the board. Swap J1 with J3 and swap J2 with J4. Then power up and test each switch again.

No idea if you need to remove Sparky, but you should probably go ahead and order an opto. They come with the wire and connector attached. No soldering. Get a spare for later.

It could just be a bad solder connection on the board, but it could also be a cracked leg on the LED, which isn't easily fixable. Metallica uses transceivers, so same part number on both sides. Part number is in the manual.