Similar experience here:
13" MacBook Pro with Mojave (10.14.1) with all updates applied
BLE connection to: Tac X Neo & Wahoo TICKR HRM, latest firmware on both
Zwift game version 1.0.31423
Companion App on Samsung Galaxy S8+
Doing workouts, resistance works initially with ERG mode on then drops off, similar when just riding, hill resistance stops working after a while. The wattage comes through consistently, it’s only the controlling of the resistance that ceases to function.
For instance on the McCarthy Special, the resistance works as expected during the warm up and the first three intervals, however fails to decrease the resistance following the third interval. Attempting to disable and re-enable ERG mode while still in the workout doesn’t resolve the issue.
I was going to purchase an Apple tv 4k, however given these issues, I think I’ll hold off for now.
I done another spin yesterday evening (The McCarthy Special) and when using Ant+ to connect the Neo and TICKR to Zwift on the MacBook Pro, everything worked as expected.