Diagnosis
That's HDMI-CEC (Consumer Electronics Control) acting up. It's the feature that lets one device control another over the HDMI cable, and it can get overly chatty.
How to Fix It
You need to disable CEC on the PL1. 1) Press the Settings button on the remote. 2) Navigate to 'Advanced Settings' or 'Device Preferences'. 3) Look for 'HDMI CEC' or sometimes 'AnyNet+' or 'SIMPLINK' (different brands use different names). 4) Turn it OFF. This will stop the projector from sending or responding to power and volume commands through the HDMI cable.
If That Didn't Work
If the phantom power cycles continue, the issue might be originating from your connected device (like a cable box or game console). Try turning off CEC on those devices as well. As a test, simply disconnect all HDMI cables when not in use. If the behavior stops, you've confirmed the source.
Heads Up
Turning off CEC means you'll lose the convenience of one remote controlling everything. You'll have to use the PL1 remote for the projector and your soundbar remote for volume separately. It's a trade-off for sanity.