Using Ubuntu 20.04.2 and gpsd 3.20.
I replaced the long running USB GPS with a RS-232 device, reconfigured and rebooted.
NMEA data is just fine and ppscheck works but gpsd is not processing the PPS signal on CTS.
ppstest and ppswatch fail, xgps does not show PPS, and ntpshmmon shows only NTP0, i.e. not NTP1 and NTP2 as one would expect.
I ran gpsd as a regular user with -D3 -N to look for errors but it looked OK and was accepting PPS.
ntpshmmon showed NTP2 and NTP3 and xgps did show PPS.
So the problem is not the hardware.
Guessing it was some sort of permissions issue I had introduced over the years, I did an apt purge gpsd, apt install gpsd and started fresh with a reboot.
Thinking maybe apparmor was doing something, I stopped it and did a teardown.
# Other options you want to pass to gpsd
GPSD_OPTIONS="-n -b -s9600 "
Now I have no clue where to go next...