Damn that’s crazy! I have a low heart rate as well sometimes. I used to do some great running a few years ago and plus years of skateboarding and stuff, I think my heart might be a little more accustomed to running at a slower beat. One appointment I had I think a 45 bpm. The nurse took it again and asked me if I exercised a lot lol she looked nervous.
Hopefully you are just more fit than others! 40 is a little lower than I would say is decent but not terrible. If it is something, at least they found it during a routine thing instead of you passing out!