It occurs to me that there is a minor chance but one that’s easy to try, that the Join EUI is causing issues somewhere deep in the heart of the stack. We know that all zeros as per the LoRaWAN Alliance docs and the hint on the console causes issues with LMIC. I use authorised EUI’s so I’ve never tried all zeros or any other repeating pattern but we have debugged similar on here on several occasions. So it may be worth a try - start creating a device, grab the generated EUI, cancel / delete and then use that for a Join EUI.