Suddenly losing resistance

Are you using Bluetooth or ANT+?. It sounds like your signal is dropping. Try one or the other if you have the hardware capability.

I can see this thread is a month old, nonetheless here's my 5c:

I had what appears to be the same resistance issue with an almost brand new unit. As with OP, no strange smells or total resistance loss. Resistance increases slightly when incline increases, still, I can spin merrily along going up 8% which unfortunately is totally unrealistic . The speed shown in the app is realistic or even a bit on the low side. 

Went back and forth with support, provided files etc. During this, the unit started creaking loudly especially after some time on the bike. Sent video with sound and Garmin proposed a replacement. This was  in December, two months after the original ticket. They told me to expect the unit the following week. Shortly after, the resistance issue disappeared while the creaking remained. 

Fast forward to now. I get a weekly mail stating that my new unit is backordered. A few mails revealed that they hope to deliver in March. Unfortunately, the resistance issue is now back. Tried with Tacx' own app, Sufferfest, Rouvy, Zwift. Tried with BT, ANT and both. On Mac, iPad and iPhone. Turned unit on and off. Reinstalled apps. Factory reset the unit. Turned Mac on and off. Ensured that no devices were connected by BT, even turned them off.

As with OP, no strange smells or total resistance loss. Resistance increases slightly when incline increases, still, I can spin merrily along going up 8% which unfortunately is totally unrealistic . The speed shown in the app is realistic or even a bit on the low side. 

Anybody have similar experience? I am grasping at straws to extend the life of this unit until the outdoor season starts or a new unit arrrives - which ever happens first.

I'm having the same issue.  My Flux S trainer used to have good resistance in Zwift but now it is the same regardless of whether I'm going uphill, downhill or flat.  I'm reaching out to Garmin for service.  

You need to make sure that the trainer is paired as a controllable trainer in Zwift, also that trainer difficulty is set above 0 (above 10% at least for even subtle changes to be felt) and that there is not another device fighting for control of the trainer at the same time. 

Thanks for the suggestion. I’ve put 84 hours in on zwift and this is a new issue. I tried moving the trainer resistance above 50 percent but it doesn’t help. I haven’t changed any other settings. Maybe I’ll try resetting the unit and zwift. 

Are you certain that you don't have a second device with a connection to the trainer?

For example, I run Zwift on Windows and use ANT+for connections. My fenix 6X can also control the trainer, as can the Tacx apps on my phone. The Tacx apps (Android) are especially tenacious at locking a grip on the trainer. They need to be force closed and/or Bluetooth turned off on my phone to release control.

If I start Zwift before I start Indoor Ride on the watch then the watch seems to respect the control from Zwift that already exists. If I start Indoor Ride on the watch before Zwift has gained control of the trainer then there is a conflict. 

My experience is that if you have two devices fighting for control then the resistance can be unpredictable.