Ok, this is not a Zwift Companion App issue and it is definitely not a zwiftpower.com issue, this is an internet connection issue. The issue could be with your ISP.
I think you are getting confused on the way the Zwift activities get uploaded.
If there is an internet issue near the end of your activity or during the update process the .fit file will not be uploaded since it will be malformed.
Again, Zwift activities DON’T get uploaded to the ZCA, the ZCA only reads what is uploaded to the Zwift servers.
This is an internet connection issue either at your house or at your ISP. My guess is your ISP since the issue just started.
Have you tried restarting both your modem and router (if they are separate devices).