Hi, @LiS9 , if you have not already done so, I would recommend giving your Fitbit a restart . You may have to try this a couple of times as a single restart does not always work.
Please also try a change of clock face. You do this in the app and the instructions are here . This troubleshooting step can often reset screen glitches. If changing the clock face solves the issue for you then you can return to your previously chosen clock face if you prefer.
Good luck and let us know how it goes. Welcome to the Community Forums!
Sense, Charge 5, Inspire 2; iOS and Android