Is there a bug with body battery and the breathwork activity for the Garmin vivoactive 5?

Strange indeed. Could you post a screenshot of the Body Battery graph for that day?

sure, it looks like this:

On my mind, unless you see it every time you do a Breathwork activity, I would tell that the watch crashed and rebooted, losing the BB state in the process. If you have a PC, connect the watch over the USB cable to a computer, and look into the folder //GARMIN/Debug/. If there is a file err_log.txt or err_log.bak, open it with Notepad, and check whether the date and time of the crash corresponds to the time of the activity.

Thank you, this is helpful! It happened 2 out of 3 times I used the activity, but one time it was completely normal. I currently don't have my USB adapter with me, but will do as you suggested next week.

... and BTW, how does the Stress graph of the Breathwork activity look like?

This is the stress graph from the activity and the general one from that part of the day. 
The breathwork activity does vibrate every 4-8secs, and I am sure that I would have noticed missing more than 1-2 vibrations, but it’s absolutely possible that it rebooted this quickly. 

Nothing really suspicious there. The stress is a bit high for a relaxing exercise, but not unusual. There are also no gaps. So either the watch crashed and rebooted, losing some data, or there is indeed a bug in the Breathwork activity of Vivoactive 5. I recommend asking on the Vivoactive 5 forum, whether other users of that model have similar problems too.