I think this is not an typical R#176 bug. I had these missing numbers in R#174, too. It could be my imagination but I think it has something to do with objects and (invisible) walls blocking the line of sight. I know that sounds stupid because you can see DMG-Numbers through walls but this bug happens especially with enemies that stand right around the corner or exactly at the corner (like cave at CRP) and when there are some trees between me and the mob (wastelands).

I confirmed too, that I do damage but the numbers are missing.

A wild guess would be that the number of (invisible) walls/objects in line of sight are the relevant factor here.