Maybe it is a loose connection.
The connector to the "Mode sign" might not be sitting well on the board or with whatever it is connected.
Just press the connector a bit (with the game turned off) and see if it works then.

If that isn't the case I could also imagine, that maybe the wires in this connector jumped out a bit and thus there is a loose connection.
I had that once on my Attack from Mars, the saucer didn't lite up. One of the wires completely jumped out of the connector.
I had to crimp these wires back in.

According to the TSPP manual the connector is located on the board behind the "Mini-TV", if you've meant the little display-screen.

I'm having the same issue with the Led Mode sign. Did you solve the problem??

Similar problem came up in this thread: https://pinside.com/pinball/forum/topic/tspp-leds-on-mode-sign-are-acting-up

The short of it:
-The flickering (in that case) looked like strobing of the lamp matrix
-One of the LEDs was the problem. Once it was removed, the problem went away.
-A replacement LED has been ordered (VQA29 Miniature LED TRIANGULAR GREEN). Hopefully that will work as a replacement.

Is there a website you know that sells this replacement LED? I bought one online and it is the right size but after replacing it the LED stays lit all the time.

I don't -- I was helping someone in the other thread diagnose the problem. Given that the resistor is 220 ohm, I was guessing at the correct LED to use (and that was the only triangle shaped one I could find). Have you tried the "regular" 3mm and 5mm LEDs that are often sold in Arduino starter kits? They may be a good thing to try and see if they behave correctly. Then based on their specs, see if you can find one with triangle shaped head.

Digging some more at digikey, this looks like the a good replacement: https://www.digikey.com/catalog/en/partgroup/4-0mm-x-4-5mm-triangle-with-flat-top-led-lamp/35917. It has looks like it can handle 20mA, with an input voltage of 2.2V. Using this resistor calculator: https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-led-series-resistor and assuming an input voltage of 6.3V, the 220 ohm resistor is perfect. (But keep in mind that I am just guessing at the size of the LED.)

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