Don't feel bad, at first, I was thinking the same thing :)

Because I just reloaded all of my Pi devices from scratch, I'm unsure if this is an "issue" with the config of the serial device on-board or not quite yet. All I know is that after I built my PCB hard-soldered test platform with all my ICs and sensors I'm testing, but before the OS upgrade, it worked without hiccup.

No software changes to the serial code on my end (or my underlying C code), so I'm thinking it has to be how the device is configured hardware-wise.