axi/axi-stream/rtl/AxiStreamTap.vhd appears to never "tap" the TDEST requested.  Looking at the code, it depends upon AxiStreamMux to route the TDEST to a separate axi-stream master, but that routing always chooses the non-tap route.  This is because AxiStreamDeMux searches the routes in ascending order, and the first entry in the routing table is a catchall.

https://github.com/slaclab/surf/blob/a8b3b3312f0816e804ec53d9f3cd220244234c32/axi/axi-stream/rtl/AxiStreamTap.vhd#L51-L65