Been busy so only just getting around to looking into this again.
Built 0.183 from scratch; makefile was unaltered. The issue does not seem to be repeating itself in this version, either from commandline or in QMC2.
System environment is pretty much unchanged except for application updates, but I doubt that they would have had anything to do with it.
I'm going to try running off a few more builds with some modifications to the makefile, but things are looking up.