Overlapping transparent sibbling with Text item destroys batching

We detected a performance degradation in our application since we moved from Qt 5.12.12 to Qt 5.15.8

I could narrow it down to a simple example that seems to break batching in the scenegraph thus leading to more drawcalls. It seems to happen as soon as a Text item overlaps with something that has transparency (e.g. an Image)

See the attached example for reproduction. The white (or half transparent) rectangle grows and as soon as it touches the sibling Text element none of them are any longer batched together (as seen with QSG_VISUALIZE=batches). When the rectangle is drawn over the text element (the last two rows) the batching seems to fail for the rectangles.

In Qt 5.12.12 the batching works like you would expect. But Qt 5.15.8 (and also Qt 6.4.3) shows the broken batching.