beginTransparencyGroup

ISO 32000-1, 11.4.5: a group's constant alpha and blend mode apply once, when the group composites onto its backdrop. So a group that has them paints into a layer first (#77). An isolated group's layer starts transparent. A non-isolated group's layer starts as a copy of the backdrop, so a paint inside blends with the page, and at the end the layer mixes with the page by alpha. That is exact over an opaque backdrop. A non-isolated group with a blend mode of its own starts transparent, which is exact when no paint inside blends (#125).

In a knockout group (11.4.6) each paint replaces what lies under it inside its shape, so each paint composites against the group's transparent backdrop. That is the result of the spec unless a paint inside blends. A group nested in a knockout group gets a layer, so its own paints do not knock each other out.