I'm running into what seems like the same bug on firmware 23.20 that was corrected a year ago here:
forums.garmin.com/.../broadcast-heart-rate-drops-every-few-minutes
I believe this started again earlier than 23.20, but I don't know the specific version as I don't use the feature often.
To reproduce:
1) Scroll to the Heartrate glance on the watch, press start
2) Press start again, then Broadcast HR
3) Press start one more time to start broadcasting
4) Connect via Bluetooth to something like Zwift's Companion phone app
After ~5 minutes the connection will drop, turning off/on boardcasting on the watch and reconnecting on Zwift brings it back. At first while working the watch's screen lists my phone name at the bottom, that disappears when it disconnects.
It's worth saying that this also happens if broadcasting HR from my watch's glance view to my Wahoo ELMENT Bolt head unit, it works at first and then disconnects after a few minutes, it's not specific to Zwift.
Also, as with the bug report listed above that had been fixed, broadcast works fine and does not disconnect if I instead start a "indoor bike" activity and just have it broadcasting HR to Zwift or my headunit.
