Managed to reach the HW engineer and I have a definitive answer now.
PicoPix Max only accepts 15V.
Due to thermal and efficiency concerns at this size, some parts inside have a max limit of 15V. 20V parts could not be used.
PD optionally allows 15V @ 4A instead of 3A, if local optimisation is of value. Our engineer believes PPM is the perfect candidate for ‘local optimisation’ as 15V4A has better thermal+efficiency characteristics than 20V3A.
The supplied power adapter supports this optional 4A output at 15V so it can run the projector at full brightness and even charge a bit. But it seems the majority of 3rd party PD adapters only support the minimum requirement, which is 3A. That’s why these adapters will charge the PicoPix when off, but will probably struggle during usage in Presentation mode.
If you can find a PD charger that does output 4 amp, that’ll be a better fit. But otherwise 15V at 3A will charge the projector when off, and should power the projector fine at eco and likely also normal mode.