I’ve experienced this problem many times.
When I ride into Zwift in simulation mode, the hill resistance comes in late, like 2 or 3 minutes after I passed the hill. It comes when I’m on a flat part.
This is very annoying and I don’t know why it happens.
I’ve also experimented some power stuck during a workout session on Erg mode. Or minutes delay, like before…
Zwift is running on an iMac on MacOS 10.15, a Tacx Flux Smart connect by Bluetooth.