If the Freeflow doesn't get the job, it isn't the driver, its the port.
Reinstall the driver on the Mac, and this time don't pick it from the list of printers, add it by its IP address, choose HP Jet Direct as the port type and choose the Freeflow driver in the Print using dropdown.
Did you completely remove the old driver after upgrading to Catalina before installing the new one? If so, I'd check to see if there's anything useful in the CUPS error logs for the printer in question upon job submission. I don't have this particular model printer so I'm unable to test it at the moment.
This is the driver that I used, but the FreeFlow receives nothing. It does still print from the other macs without Catalina. Any settings I perhaps need to change?
I have upgraded my operating system from Mojave to Catalina and now my Xerx 550 is not printing? What can I do?