Looks like I may have resolved the issue.
I loaded up the https://github.com/disk91/WioLoRaWANFieldTester project with Arduino using the .ino file.
That loaded and ran fine.
Then I tested the uf2 Bootloader and that now works as expected (tested on Ubuntu).
So it seems that there may be a problem with the WIO Terminal fimware on early shipment devices that prevents the uf2 bootloader from working.
But the solution is easy - just load up a (suitable) project that supplies the Arduino files and it should update whatever is causing the problem with the uf2 Bootloader.
I used the .ino file from this project:
This project offers both the .ino source and the uf2 options.
Using the .ino upload from other projects that did not also have the uf2 option did not fix the uf2 bootloader issue.