As suggested by @jkbonfield in #715 (comment):

as an aside, I note you're also using MAPQ of 255 for "unavailable". Commendable, but my experience is that everyone just uses 0 with unmapped data. I think this is because when FLAG 4 is set the specification states no assumption can be made about MAPQ, so it just feels cleaner to zero it out as all other fields have been.