Ah, @htdvisser indeed wrote on Slack in August 24th:
At the moment we only piggy-back ADR on application downlink, but we will indeed send the ADR command sooner. Even better, if the margin is really big, we might already send a first ADR command immediately after join. See Send device straight to a higher SF on join if possible · Issue #477 · TheThingsArchive/ttn · GitHub
Ah, and here on the forum, September 11th:
So you’d either need to wait a bit, or somehow set the ADRACKReq
bit every now and then (what LoRa module is in the RisingHF USB modem?), or somehow make the application schedule a dummy downlink if it sees that the node is on a low data rate…? And maybe disabling link check can stop the node from going to SF12 to start with (I don’t know)?
(And file a bug with RisingHF.)