Overlapping zones - is there a fallback rule?

I’ve always been under the impression that when zones overlap on the same layer, the one with the higher priority “keeps-out” the lower priority zone for the overlapping region. As @Rene_Poschl puts it, the higher priority zone gets “exclusive access”:

So I’m at a loss to explain this behaviour:

On the bottom layer of this board I have behaviour that matches my expectations. The higher priority zone within the surrounding lower priority ground pour leaves the circled region unfilled. Makes sense if the higher priority zone has exclusive access but is unable to fill it.

But on the top layer on the same board, the lower priority surrounding ground pour fills a gap in the middle of the higher priority zone.

In both screenshots the ground pour is selected, so the relevant regions appear in the lighter shade.

I believe this effect is not just visual, since I only noticed it when I was reviewing the gerbers.

All I’ve done so far is a lot of double checking priorities, re-filling zones and generally just poking it to see if the effect is stable, but haven’t yet figured out the root cause. Before I get too carried away chasing rainbows or constructing bug reports, what is the expected behaviour here? I actually don’t mind either behaviour, but certainly need it to be predictable.

FWIW, I’m trying to replicate an Altium project, where the behaviour is like my second the second example - the zones get “shared custody” of the overlapping region.

I reported such behavior few months ago with almost the same drawing as your first one. If I were at PC with my mails I would have a chance to find it, but I’m not.
The reason was very fast deduced by developers.
At your first picture the higher priority zone when filled has also filled the part to the right od D_N and D_P when the less priority zone is filled. Then this not connected island is removed.
If you move D_N to right enough to not left there a space for filling then the less priority zone will be closer.
As I understood, now it can not be changed - there are important rationales for first filling less priority zones and then delete isolated islands of higher priority zones.
But it was assumed as a bug to be considered in future.

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Ah! Makes perfect sense, and the test you suggest behaves just as you stated. Thank you!

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