I noticed exactly the same problem a couple of weeks ago, but mine also affects the 2,3,4 LEDs on the rev counter as well as temp gauge.
At first I thought it may have been a dodgy connector, so tried to push various points of the dash in, but after some research it appears like a fault between the LEDs and the circuit board - they start to come away and loose connectivity.
There are two solutions, as far as I can tell -
1) Buy a used (or new) dashboard, but you need to pay Audi to code it to your car (because of mileage tampering). Whether someone on here can / will do it cheaper I don't know, but people were talking of very expensive fixes through Audi themselves (shock!)
2) There guys were recommended:
https://cartronix.co.uk/product/audi-a4-full-colour-lcd-replacement-repair/
They will repair your dash, and include a lifetime warranty. I know the page above is talking about the LCD screen repair, but from what I've read people have also used them to repair the dash lights as well.
Oddly, mine has been behaving itself recently so I've not done anything about it, but it will only get worse and when it does I'll go for option 2. It's a bit of hassle driving to them but at least I know there is a lifetime warranty included.
Hope that helps