Hi Vincent,
I am dealing with the same annoying issue. Forget about finding a solution at the Zwift or Tacx app settings. This issue is caused by the Garmin FR945 external sensor settings.
Garmin FR945 interferes the ZWIFT ERG controll. As you, I am also recording my workouts with the Garmin FR945 in addition to Zwift to get the Garmin FR945 metrics.
Sometimes the Garmin FR945 takes controll and prevents Zwift from changing resistance. Could be depending on which device (Zwift ipad or Garmin FR945) fist connects to the trainer. Do not know.
The only workaround I found, is to switch off the Garmin FR945 and turn it on only after I started the ZWIFT workout and checked that the resistance control works. A limiting factor to this workaround: if you need to make changes to the Zwift resistance settings during your workout (e.g. lowerering the target watts niveau of your planned workout), the Garmin FR945 again comes in your way and prevents this change. I needed to power off my FR945 several times lately, then waited for Zwift/ipad for taking over controll again. Very annoying, indeed.
UPDATE: issue could be cause because I added the TACX trainer as a TRAINER sensor type to my Garmin FR945. I will remove it and try to add the trainer as a Powermeter and Speed/Cadence sensor. This will do it.