DNP Malformed Packets and Write, Unknown

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Could this be a dissector error in Wireshark

Yes, it could be.

or is this a true protocol implementation error in the outstation?

It could also be a protocol implementation error.

Submit an issue on the Wireshark issue list, and attach the trace file (pcap/pcapng/etc., not a screenshot) with enough packets in it to show the problem. You can check the "This issue is confidential and should only be visible to team members with at least Reporter access." checkbox if there's information in the packet trace that shouldn't be made public. That way, we can look at the trace to see what the problem is and, if it's a dissector error, figure out how to fix it, and then test the fix by using the trace.