is AU915 constrained wrt on air dwell time in the same way as US915 or have you perhaps misconfigured install such as to effect such a limit? With US915 and (IIRC) a 400ms dwell time limit LoRaWAN Is essentially unusable beyond SF10 and even at SF10 message length is constrained compared to other territories - (again IIRC) a limit around 10/12 byte payloads.
How big is your payload and is system configured correctly or constrained by regulation? In reality it would be the transmitter that is constrained (the node in this case) but you say you see message arrive at GW - it may then be the backend rejecting as “illegal” message.
Another option might be if signal marginal or at limits of channel frequency/poorly set up/matched then message integrity checks might be failing on longer messages which would cause GW to possiby not pass received message on due to MIC fails!
WIthout going to check I dont have direct answers but just flagging options - also is AU915SB1 the usual recommended plan (with GW & Node freq/channel options all correctly set up & matched?)
Late here, a bit tired and just thinking aloud wrt possibilities!
The other common thing to look at is all appropriate keys (DEV EUI, APP EUI etc.) entered correctly - if message at GW but not at application one of the most common problems is mis-match between how App is set up and actual keys used…though fact you suggest working ok for DR2-> DR5) suggests that isnt problem here…make sure node & GW have sufficient seperation to ensure no overload at margins of longer messages…