A lot of synths have this ability.  However I figured out that changing the polyphony to 2 solves the problem and allows the envelopes to retrigger.

Maybe if you boost the sustain level of the Amp EG a bit it will work better with solo.

abacab
Can you point out any synth examples that do re-trigger envelopes in solo mode?

Roland Jp-8080 has mono and legato modes. Mono re-triggers the EG and legato doesn't. On Dave Smith's Evolver EG re-trigger is selectable. Omnisphere has solo and legato modes - solo makes the patch monphonic and the EGs retrigger and if you also select legato the EGs won't retrigger.

I noticed that if you have the sustain level for the amp EG set too low, the level will fade as you play legato.  But if you raise the sustain level some, you can keep playing without needing to re-trigger a new note on envelope.