It is quite possible the crank was not fully seated in the splines and loosened because of this. An inspection of the splines on the crank
should show if this was the case: the splines will be partly 'boogered up'. If so just take a knife and carefully trim off any loose shards
of metal and next time tighten them more firmly than you think necessary and then some. I have had this happen in the octalink era.
Also had it happen with 8-9-10 spd cassette lock rings, until I got a torque wrench.
Unlike the era of sq taper cranks and dodgy metallurgy in the crank fixing bolts, when it was possible to pop the head off the fixing
bolt, Shimano fixing bolts are stout enough to take any practical torque you can place on them. But a scale approach can give you
an idea of where you are at torque wise which should be in the 30-35 ft lb range.
Thank you for your post. I'll inspect it tomorrow in the daylight. I'll keep in mind what you said, and hopefully it'll turn out alright.