Krita and Gimp rendering of Opacity on passthrough groups differs from the standard definition and MyPaint
Under the definition here: https://www.openraster.org/baseline/layer-stack-spec.html#compositing-the-image it is stated that while groups have opacity less than 1.0 (100%), they should always be rendered as isolated groups. However, both Gimp and Krita allow opacity of less than 100% while maintaining the group's pass through mode. (and the mergedimage.png which is generated when saving an ora maintains the same look) MyPaint seems to attempt to follow the defined standard by locking opacity at 1.0 while a group is in pass-through mode.
I think that the standard may perhaps be updated to allow less than 1.0 opacity on non-isolated groups, as both Krita and Gimp seem to have a working implementation that defines how this rendering should take place.