Some more information here–I have two Model01’s (one for home and one for work), and I am unable to flash either, so I suspect that the issue with flashing is macOS Catalina-related.
Some searching turned up this thread:
Which discusses problems with Catalina and the port-probing that happens before the flash.
Furthermore, I took the failing Model01 home and it worked fine for a short time, then the right half stopped again. I will swap keyboards and take the other to work tomorrow and see if it starts to behave strangely.