Same issue for me started today, still looking for a solution!
I use a 2018 Kickr Core with Zwift on MacOS. On my free rides today (Saturday, 20 April) I noticed several occasions when the gradient in Zwift changed but the resistance on the Kickr did not immediately increase/decrease. The problem appears intermittent as there are times when the trainer resistance changed immediately with the gradient. When the lags occurred, it happened for both power increases as well as decreases, and the duration of the lag appeared variable.
I followed all troubleshooting on Wahoo’s and other websites including removing all other Bluetooth devices/connections, “forgetting” the Kickr in the Wahoo app, unplugging the Kickr Core, restarting Zwift, rebooting the computer, trying a second computer, and performing spindowns.
Spindowns in Zwift consistently fail at the very end with a “connection timeout” error. Even though the trainer flywheel stopped rotating, Zwift showed 2 mph. Spindowns through the Wahoo app completed successfully, but these finished before the trainer ever got below 5 mph.
The Bluetooth light was solid (not blinking) during most of the rides. I read on a forum post that the light is supposed to blink when data is being exchanged between the controlling app (i.e. Zwift) and the trainer. Zwift always immediately registered increases and decreases in power output whenever I pedaled harder/faster or easier/slower.
Finally, I tried a simple workout in the Wahoo app using all four modes (i.e. free ride, resistance, erg, and sim). As best as I can tell the trainer responded appropriately in all cases.