Hi, I had the same issue on a windows laptop with kickr core (but the issue didn’t occur running through iPhone).
Zwift helped me find a solution.
Turns out Virtual Shifting was enabled on my laptop, despite there being no option to switch it off!!
The only way is to manually edit your ‘prefs.xml’ file in your zwift folder.
“ check against your prefs file, you can open the file for see if the line <VIRTUAL_SHIFTING_ENABLED>0</VIRTUAL_SHIFTING_ENABLED> has either a 0 (meaning disabled) or a 1 (meaning enabled) in it.”
Or Zwift can tweak and retune your prefs file if you email it to their support team.
I think this all started for me when the virtual shifting update was rolled out in Feb 2024 ish.
Such a joy to ride with no more rodeo resistance fluctuation!
Tom VegMonster