I decided to switch to shift registers to read in the 12 VFD segment lines to recycle some of the IO pins for future SPI peripherals. Unfortunately due to Covid, Digikey, and Mouser shipments have been heavily impacted, and receiving the 74HC166 registers I ordered took weeks. Ditto for the replacements when the first two I wired in didn't work.
I switched to 74HC165 registers when the replacement 74HC166 registers didn't work. The 74HC165 registers work great, and I still don't know why the 74HC166s wouldn't work. I'm happy I'm seeing expected data again, though.