Well, you can’t fault Seeed’s support! Just got off an hour-long video call with an engineer, who went through all the possible software-related things it could be: we checked all the configs, we changed the firmware, we set a manual LoRa frequency override, we peered at the serial logs, we upgraded the bootloader, we factory-reset them, we reinstalled the firmware, and… yeah, still nothing.
Replacements are on their way, though - and the engineer is very excited to get his hands on the faulty unit to figure out what the hell is going on, as apparently (and entirely typically) I’m literally the first person in the history of history to have this problem!