I used a half-dozen LEDs in my Stern Lightning with original boards but they did a much much faster on/off flicker. The resistor trick across the sockets fixed it. Yours flicker comparatively much slower. You might try just touching a 1.5 K resistor across one socket and see if changes anything. When I added a few LEDs to my early 80s Wms and Bally games from around that era, never had any flickering effect.

And when they turn on during play and do that, or during attract, the flashing is exactly the same, that steady even blinking? But hmm if all your other control lamps are LEDs too it's probably not like my Lightning problem after all. Weird that it is so evenly on/off/on/off like a special flashing LED would do.

Found the video snip I took of my Lightning before I added the shunting resistors. They flickered badly, except for one which did indeed blink three times during each flash in the lamp test like in your video.