MIDI portamento mapping not shown

I’m using FluidSynth with a wind controller.

I use modulation (CC1) for vibrato via bite on a few presets. When using a trombone, I’d like the bite to give portamento, so I’ve put in a mapping from CC1 to CC37 (CC5 has too strong an effect on FS), and that works. However, on the control screen when I select portamento the input CC37 (from the mapping) is not shown.

If I put in real portamento commands (CC5 or CC37), they ARE shown on the control screen.

Is this a bug, or am I confused?