My trainer does not register elevation changes or changes resistance at wrong times in game.
This only happens when I use companion app on IOS 14.4 iPhone XR with MacBook Pro Big Sur 11.2. When I have logged in directly to my MacBook and connect through its bluetooth, it works fine.
I can often fix this issue when using the Companion app, by stopping in game, unpairing, disconnecting trainer from power source, and connecting again. After that it often works (but not always). Everything else in game works fine.
I tried it with a Saris trainer and have the same issue - but the reconnect option I describe above does not work.
My wife has the same setup just a an iPad, same Tacx trainer and it works fine.