Greetings Michael
Did the display fail suddenly or did it slowly fade away?
A possible explanation might be that the LCD back light is not working properly. Some types of LCD back lights (e.g electroluminescent panel) use a voltage inverter circuit to generate higher voltages required to make the back light function.
It's possible that when the UHF is transmitting the radiated energy is able to "trigger" the oscillator circuit that drives the inverter back into life and you have a working LCD display. A 5 watt UHF set will easily cause electronic equipment to malfunction when used nearby.
The original faulty inverter circuit might be caused by bad earth or signal connection (somewhere).
You may be lucky and find an electrical connector somewhere on the bike is corroded or not connected properly (and this could be anywhere along wiring loom, but in this case I think it's more likely to be an internal connection inside the instrument cluster and that would involve taking the instrument cluster apart with no guarantee of finding an obvious problem.
I'm guessing that BMW will say that the instrument cluster will need to be replaced rather than repaired.
Please use the appropriate grain of salt on my comments
Good Luck
Ian J
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