No, can't say I have. Does your SSID disappear for some time, or is the 5GHz setting turning off permanently?

I've read about routers disabling 5GHz when they detect RADAR, because 5GHz can interfere with that use, and RADAR has higher priority. Here's an article on this, official from the manufacturer and for your specific model (another point for them in my book):

https://avm.de/service/wissensdatenbank/dok/FRITZ-Box-7530-A...