Hmmm… You will feel a difference riding a “10% slope” between workout and free ride whether or not you were using ERG for the workout, since in general workout mode artificially flattens the route.
There was a suggestion by Mike_Rowe on another thread a couple weeks ago, which I think you saw—but I don’t know if you’ve been doing this or not (?)
…do … a spindown with the Wahoo Utility app on your phone. Then disconnect from your phone, force close the Wahoo app, and fire up Zwift. Do that about once every 2 weeks
(referring to calibrating your Kickr—not sure if it’s a solution)
Generally, are you making sure to quit the app every time you ride? This seems to cover a few seemingly random quirks. Otherwise either there is something obvious that is being overlooked, or it’s a conundrum.