Please put it back to DEFCON 1.5 
I’m having the same issue 
Here are my observations so far :
- after updating Zwift to 1.34 I did only free rides for a few days without issue, the handlebar buttons on my up to date Kickr Bike v1 were doing great as always (power-up, u-turn and left/right lane)
- then on Tuesday 18th I join a Cobble Crusher race in which steering was disabled, but when I tried to activate my PUs with the handlebar button, no effect; I had to use Companion
- once the race had ended, I stayed on the course and the steering icon appeared again on the screen but without effect, the buttons were not working (PU, U-turn and steering)
- I ended the activity and started a new one in free ride without quitting Zwift, same result, the handlebar buttons were not working
- I also tried disconnecting every device on the pairing screen with no effect, the buttons were not working
- I stopped Zwift, started it again, launched an activity in free ride and the buttons were working again
- On the following days I only did free rides and the buttons were working
- Then today I joined a Cobble Crusher race and same as last Tuesday, impossible to activate a PU with the handlebar button
- Once the race had ended, the steering icon was back but the buttons were not working
My guess is that, once someone with a Kickr Bike joins an activity where steering is disabled, it basically breaks all the Kickr Bike’s buttons for the current Zwift process (not activity).
The only way to get them back is to quit Zwift completely and start it again.
Hopefully those information will help the devs finding the source of the issue and we’ll have a fix soon. With the Crit series starting next week, the Power-Up button is crucial!