I have written some test ROMs from scratch, that are designed to run in the game. Give them a try and see if they help you find the problem.

You can download them from here: http://pincoder.reversion.ca

There is also a thread on this site where you can leave some feedback: https://pinside.com/pinball/forum/topic/new-williams-system-6-in-game-test-roms

Let me know how you make out!

It may come down to how long the bulb is being fired.. Lamps in the matrix are never continuously on, so diagnostics could be firing them for longer, while attract/game mode could be slightly shorter. The difference could be very little, and perhaps the driver board transistors for that bulb are not reacting in time. Try swapping wires with the bonus 10K light and see if you get the same result.