For the Zwift Hub versions of that trainer, they use different power output spec to what you are using - using wrong spec output could cause strange things to happen so I would suggest you contact Jetblack and ask them for correct spec if you cant find online and DONT use the one you have otherwise you could cause permanent issues with your trainer.
Zwift hub version - from DC Rainmaker
Grinding could be one (or more) of a few things like indexing gears, spacer installed when it shouldnt be (or vice versa). Do a search as this has popped up quite a bit with Zwift Hub version.