This is a new very recent bug. Either brought about by a Zwift update or an iOS update.
When riding in non-ERG mode (for example, just ride), the gear shifter doesn’t seem to work. It shows Gear 4 and then when you change it up to 5, it shows Gear 5 for about 2 seconds and then shows Gear 4 again. As you continue to go up the gears, it toggles between say gear 12 and gear 4. Same thing if you are below gear 4. As you go down from gear 4 it shows gear 2 for a small amount of time and then shows gear 4 again.
This is a bug on the two identical Wattbike Atoms I have.
There was an intermittent problem with the ERG mode but that seems to have been fixed by rebooting the iPad.
Zwift on iPad v 1.0.35413
Wattbike Firmware: 1.01.83