Why does WireShark think this frame is a TCP segment of a reassembled PDU

Solution 1:

Having the hosts numbered .76 and .67 is a little bit mind-numbing.

Wireshark is calling frame 6 a "TCP segment of a reassembled PDU" because your TCP implementation on 10.10.10.67 is opting to send an ACK w/o payload (a "naked" ACK) rather than including the payload that gets sent in frame 6 w/ the ACK in frame 5. (This is an OS/IP stack-dependent behavior.) This is, in turn, triggering a behavior in the TCP dissector to hand off the payloads from the multiple TCP segments to the FIX dissector. For whatever reason, the FIX dissector isn't interpreting frame 6.

If you turn off the "Allow subdissector to desegment TCP streams" option in the TCP dissector's options you'll find that Wireshark interprets this differently:

Here's some discussion from the wireshark-users list about the same thing.