Glissando doesn’t play back because it’s not implemented yet.
(Yeah, I know that sounds like “it doesn’t play back because it doesn’t play back”…)
Implementing it “properly” is probably quite a big job, considering the different versions of gliss on different instruments, and the different control messages that different playback libraries use.
You can add playback playing technique to an expression map to activate those key switches, and add a playing technique to trigger the playback technique.
For string instrument portamento and glissando are different. Portamento is the unavoidable sliding along the finger board to reach the next note, if two notes on the same string are too far apart to play without shifting the left hand position, and players usually try to minimize it. On the other hand glissando is a deliberate and gradual change in pitch.
I would guess the portamento key switches in Halion implement “real” portamento, and possibly the different switches are because the transition between notes using different fingers can be partly a slide and partly a jump, if you care about simulating that level of playback detail.