Oh yes I didn't think about that. The channel adapter is added automatically because the multiplexer creates a channel signal (to indicate which source the packet came form) but the TSE's sink doesn't support channels. Now from the documentation:
--- Quote Start ---
You can also use
this adapter to connect a source with a wider channel to a sink with a narrower
channel. If the source provides data for a channel that the sink cannot receive, the data
is not transferred.
--- Quote End ---
This is rather unintuitive. The adapter isn't even configurable so you can't say what channels you want through, it will just discard the extra channels. IT looks like the only solution is to write your own adapter, or modify the existing one (but in that case be very careful that SOPC Builder/QSys doesn't overwrite your code the next time you generate!).