BASS_ChannelSetAttribute will have successfully set the volume (BASS_ATTRIB_VOL), but it won't have cancelled the slide, so the slide would almost immediately change the volume again. The reason BASS_ChannelSetAttribute doesn't currently cancel a slide is because it is actually used internally by the slide, ie. BASS_ChannelSetAttribute is called periodically (usually every 10ms) to apply the slide. Volume changes are ramped in the output mix, so the slide steps will be smoothed away.
When playing through a mixer, volume slides can also be implemented with BASS_Mixer_ChannelSetEnvelope, if you would like them to be separate from the BASS_ATTRIB_VOL setting. BASS_FX_VOLUME effects (via BASS_ChannelSetFX) is another option.