Hi Richard.
You can use the 'dftest' executable that is built along with Wireshark to convert a display filter string to the byte-code used for the Display Filter Virtual Machine. Do that for for "wlan.dmg_params.bss == 1" and compare that to the output for a similar display filter for another field that is also FT_UINT8.
I suspect they will be the same; if so, it wouldn't be an issue within the display filter code.
Is there any possibility that add_ff_dmg_params() isn't being called or isn't adding dmg_params to the proto_tree in some cases?
Gilbert