Have you looked at the RS485 line with an oscilloscope to see what's actually being sent?
The two RDM Sniffers I know of are the Enttec and the DMXter4.
In general using auto-direction control is a bad idea with RDM. The timing requirements are pretty tight, and you need to have control over when you assert TX-enable. I don't know the specific auto-direction implementation you're using, but they often are tuned for much lower baud rates and looser timing.
(Full Disclosure: I have worked for GDC who makes the DMXter4)
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