But for the uplink it specifies SF7 to SF10, while for the downlinks it lists SF7 to SF12. Also, the uplinks have 9 frequencies, the downlinks only 8, so I can only assume that 904.6 SF8 should not be used when relying on downlinks, like when using ADR? Not a clear specification if you’d ask me. Also, US has the notion of “hybrid” which is not mentioned at all?
If that’s true, then I’d not alter my application/node but I’d look for a different test gateway, or get a full gateway.
But indeed, if the downlink has not already been scheduled then TTN might not wait long enough for the application to return a downlink. But then TTN is scheduling it for the next opportunity, not the gateway; see My application's downlink is always queued for next uplink.
(This ESP8266 test gateway worked great for me, for EU868. See also @bluejedi’s list in Single Channel Packet Forwarder part 3 [Deprecated] .)