in NEXT, when i go to render mask and set it to include/exclude or selected and hit render, then i animation i get stacked up renders which are baked together in all channels.
It used to happen in 3.6 too from time to time.
Sure this is not how its meant to be working in animation?
It looks like the VFB is not cleared and the result is saved with frame increments.
It has always been like this. If you use the memory frame buffer and want to render animation with render mask - you need to set the Previous render option to clear. It is located in Settings->System (advanced or expert view).