In response to our query

Yamaha Support confirmed that the design behavior for the front panel Portamento Time Knob on the M models is to force the time to a single value and override any part-specific settings that have been made.

This is the background that was submitted with the question:

1. You can specify a performance level offset for portamento time (-64 to +63). This offset is applied against each part setting. This offset setting is NOT affected by the front panel control.

2. You can specify a portamento time for each part individually (p.115, p.265) BUT moving the front panel knob will change this setting for all parts.

3. You can create portamento groups (p.209). Each group can have a different time AND a different 'Time Mode' (Rate 1, Time 1, Rate 2, Time 2) specified

4. You can specify 'Part Param -> Porta Time' as a control destination for a knob, pedal or other controller.

5. GOTCHA! - Moving the front panel Portamento Time switch actually changes the time setting for ALL of the parts and thus for all of the groups you just spent time setting up. This isn't an offset but changes the part values directly. Meaning that if you then store the performance you will have destroyed any custom portamento time/group settings that you made.

Would it make sense to have a System/Performance 'Portamento Override' switch that would determine if the front panel knob actually overrode the individual part settings instead of just overriding/offsetting the performance level offset?