Portamento doesn’t work in a consistent way on polyphonic patches as the synth glides to the pitch assigned to a voice from the pitch assignment for that voice. As the voices are dynamically allocated, the behaviour may seem unintuitive, even though it is (in theory) predictable. The X-Station is 8 voice polyphonic so the following example is based around 8 notes and an init patch with the portamento turned up to some degree and the amp release set to 0.:

• Play 8 simulataneous notes (C1, E1, G1, C2, D2, E2, G2, C3).  Assuming the voice assignment starts at voice 1, this will assign eg:
• C1 voice 1
• E1 voice 2
• G1 voice 3
• C2 voice 4
• D2 voice 5
• E2 voice 6
• G2 voice 7
• C3 voice 8
• Release all the notes except for the C1.
• Play and release C2 slowly 7 times and listen to the pitch change on each note on. The intuitive behaviour is that C1 is being held, so all subsequent notes should glide from the held note C1, but this logic only really works for one note held. What will actually happen is It will probably try and assign voice 2 to C2 and glide from E1 on the first note press. All subsequent presses, the voice 2 will be re-used and the glide will start from C2 and end on C2 – ie no apparent glide. Now turn the amp release up to quite a long time and repeat the test
• The glide will seem to change as the voices in the background are active until they have zero output. This makes the synth use different voices for the C2 note and each will glide from their previous note assignment, until voice 2 hits zero, at which point you’ll be back to not hearing portamento.

Because of the seemingly unpredictable way polyphonic glide works, its why we add in pre-glide as a portamento option. This gives every note on a predictable amount of glide.

If you change the voice mode to mono or mono AG, the portamento works in a consistent and predicable way. (see page 44 of the manual for how to change this).