"-b 256" seems to be working here, using the pre-compiled CONVERT-ADDONS.EXE example from the BASSenc package (C\BIN folder) to test. Please try that there, for example, like this:
./convert-addons.exe -e2 filename.wav -b 256
If that works fine, is your app including anything else (besides "-b 256") in the BASS_Encode_MP3_StartFile options, and if so, what is that and does adding it to the CONVERT-ADDONS.EXE command-line reproduce the problem?