Erg Mode Actual vs Target discrepancy

I’ve been using Erg mode for nearly a year with no issues, last week I started to be unable to hit the target power, Target would be say 115W, and at 80rpm i’d only be able to get down to 160-180W, i’d have to drop cadence to perhaps as low as 50 to get even close.
This is a Tacx Vortex.
I have:
Checked firmware and updated and calibrated before and after. no change
Used a different computer, used Ant+, BT through companion and BT direct. no change
Used a real cadence meter (80rpm didn’t feel like 80) no change.
Reinstalled zwift, no change
High wattages are fine, and this might be getting worse, initially that 160-180W for a 115 target might have been 140-150.
The actual power readings do go up and down, but obviously i’ve only got the cadence to play with, is this suggesting that the resistance is actually stuck on?

Thanks in advance.