....
After some more searching I figured out what the difference was: the "Loopback Mixing" setting in alsamixer. It's apparently disabled by default, but for some reason mine had migrated to enabled. Enabling it seems to dictate that headphone levels are controlled through the "Speaker+LO" channel instead of
the "Headphone" channel. Setting it back to disabled, and things are now back to normal. Mystery solved!
Well thanks Brendon. That started mysteriously for me a couple years ago too. But I shrugged and used the CentOS 7 machine where it worked fine. The CentOS downer was that it required closed drivers for my BMC network chip. Some days I feel like Papa Bear :-)