I do something similar now and I get 100% success rate with exercises on plan being marked complete.
- I go to fitness+ app on my iphone.
- the app has the workout for the day it wants me to complete, I tap “launch” from my iPhone in that workout
- I tap the screen share or airplay icon at the bottom right corner of the workout video before starting the workout
- i choose the Apple device I want to cast to which is my Apple tv
- i press the play button on my watch after it shares to Apple TV, and complete the workout
- i go back to my phone and click the green “done” button when the workout is finished
I've done it this way for 2 weeks now, this being the start of my third week and it hasn’t missed a workout in my plan once yet. Kinda stinks to have to do it this way, but until they fix this bug, it’s the best solution I’ve found. Initiating the workout from iPhone app and air playing it on a different device then completing it on the iPhone when finished. I also always do the suggested workout instead of choosing my own.