I know it is frustrating, keep in mind that this is a user forum and everyone answering here are volunteering their time to help others. Unfortunately, sometimes a post may just fly under the radar.
I have tested with Air Pod Pro, Bose Soundlink and Sonos Ace, and could not replicate the issue. I have also not come across another report like this.
It sounds like you also tested multiple headphones and still experienced the same issue, is that correct?
I think if the HDMI were the issue there would also be issues encountered when playing direct, not just over headphones, but it doesn't hurt to try.
Other than that, all I can think is to make sure you are running the latest OS (17.5.1), not opted in any beta, and doing a power cycle. If it persists then contact Apple support as they will be able to run through some more things with you and hopefully get to the bottom of this.
Good luck.