Hi,
I had same problem. I tried the solution deleting all the network configurations and restarting the macbook. It didn't work.
I noticed that this happened a lot while I was watching streaming, like netflix.
So, my doubt was that for some reason during the streaming (with power cable plugged in), osx was going for some reason in power saving mode.
I went to check the power saving settings, and noticed that even with the plugged power cable configuration had low settings, the same defaults for the unplugged power cable settings, something like 5 minutes screen off and 15 minutes deactivate...
By the way, I increased the time and unticked something related to the network too (I can't remember, don't have it with me right now to check what was), and this helped. The connection did not drop anymore.
Probably power saving modes deal also with network service availability, could be wanted or could be a bug in the system I don't know.
Maybe this could be related to the upgrade; maybe the installation process, apart upgrading the OS also sets some defaults that would be better not to change.
Just my two cents, I hope it helps.
Ciao.