In my opinion it's not a 5Ghz issue.
I was able to use 5Ghz band in 1.4.4 as well, with just one issue, that I'm finding also in 1.4.5:

The default hotspot, in my case, always connects to 2.5Ghz band however, I can manually force it to 5Ghz from the Network configuration window, but after saving thew new configuration the line
address1=10.250.250.1/24,10.250.250.1
is automatically overwritten by the system, and you loose the 10.250.250.1 static IP, and the Network Manager automatically chose a dynamic IP in the order if 10.42.0.1 for StellarMate AP.
The only way I solved this was to MANUALLY edit the "stellarmate" Network Configuration file adding again the deleted line, to get the static IP again.
It seems the Network Manager does not permit to set a static IP for an Access Point (IPv4 section of the configuration), so your original stellarmate configuration file is forcing it.

knro wrote: I just tested again the WiFi on v1.4.5 on both RPi3 + RPI4 and couldn't replicate any issues. Of course this doesn't mean they're not there, but from prior reports, the RPi4 WiFi is quite sensitive. Is there anything attached to the RPi4? Does this issue only affect RPi4 or also RPi3?

The change for RPI4 is the support for 5Ghz, which actually might have brought in these issues. Maybe I should make the Hotspot always default to 2.4Ghz to make it stable?