This is a particular issue between certain Tacx trainers (mostly NEO 2T) and FulGaz for Windows 10. A fix has been rolled out in the latest software update, so please ensure you have the latest version from the Microsoft App Store. In the meantime I have a "workaround" which may do the trick.
You will need to turn on the "separate power meter support" option then connect your trainer as both the trainer control source and also an external power source when pairing.
First, open your FulGaz app and un-pair your devices from the pairing screen, then continue to the main ride menu. Go into your settings (click the "cog" icon), then scroll down and select Advanced Trainer Settings. Check the tick-box next to Separate Power Meter Support.
Exit settings, shut down and re-start the app. You should then be given new pairing options when you open the app again.
Pair your Tacx NEO trainer as both trainer control AND power.
This seems to get resistance changes back for those with affected trainers.