Power reading lower in zwift than elsewhere

I am having an issue with my numbers zwift. The past few months I have been having a hard time maintaining my expected power for my long rides and just getting gassed out.

I recently did my FTP test with my power meter connected to my phone for zwift and my garmin edge 520 and found that the numbers were off by anywhere from 10-50 watts at any given moment.

I then tried an easy ride this afternoon and found that the same thing happened my zwift activity came up with an average watt of 179 where my garmin recorded 222 as my average. I did find it interesting that once I got into my FTP range I found the gap to shrink to about 10-20 watts instead of the 43 I saw in my easy ride.

I have tried to look at the stages power meter to upgrade firmware and recalibrate it, but I did a couple tests since and I am still seeing the same issue.

Any suggestion/thoughts to the cause of this?

I’ve just had the same issue. I joined a Zwift training session which I have done loads of time before and found my power output was down by about 30%