I've noticed the same issue on my Tab S7+. It will join the 5Ghz fine, but after a while it will drop to the 2.4Ghz and refuses to switch back to the 5. Sometimes turning WiFi off and on works, but most of the time it takes a reboot to get it to connect to the 5Ghz band again.
I've used the WiFi annlyzer app when this happens and when it is stuck in 24Ghz the app cannot even see the 5Ghz bands. When the tablet is working normally you can see both 2.4 and 5 in the WiFi analyzer app. It is as if the 5Ghz radio in the tablet has crashed.