To be honest I've bought a fritzbox 7590 last summer and I've had the wifi on the xa2 disconnecting randomly for months, so I'm not sure that it was broken on a recent build.
Maybe you're right (I'm new to the XA2 since two weeks or so), and maybe I've had a bit luck only, with the build of20200302, that WiFi (and especially WiFi-Direct) was stable for a few hours... but now it randomly disconnects every few minutes... either with 2.4GHz or with 5GHz too.
I tried again with the same Fritzbox,:
HTC One M7 (LOS 14.1): stable connection
HTC One M7 (LOS 17.1): some disconnects
Xperia Z C6603 (LOS 15.1): really stable connection
2nd XA2 (even LOS 16): disconnect every few minutes too
So the Fritzbox is OK (my other WiFi-Devices — PC's, TV, etc. — are running stable since months/years), and it really seems to be a bug in LOS 16 (and possible 17.1 too?).
Just my 2¢,
samhhmobil