I got my estimates a bit wrong there as I’d moved them a few times and they were about 3m away.
Bringing a different model gateway nearby or moving the sensors home where I have an IMST concentrator and Pi brings them back to SF7 using ADR. Likewise, using my ttnmapper board set to 20 second intervals nearby also allows them to lower the data rate.
I can only conclude that the TTIG doesn’t do well with ADR if it doesn’t get a packet from anything within 30 seconds. This is because it seems to do a store and forward of the received packet while it reconnects to TTN and thus will miss the downlink slot.