"Aengland gets destroyed"
That's right - I wrote my heir would get Aenglaland, this was incorrect.
There is no Bretwalda any more, so no Aenglaland. Absolutely fine and historically correct.
But: The laws of Anglia are also reset (no medium crown). I would consider this a bug. If Aenglaland changes the law, and Anglia being part does that too (and you cannot alter Anglia-law differently from Aenglaland), this new Anglia-law should remain even if Aenglaland disappears.
"you default back to standard gavelkind, and so your son inherits over your grandson."
This is fine, but should be presented correctly in the Anglia-inheritance-tab.
One could show either the Aenglaland-version with only one heir (which then would be invalidated when the Bretwalda dies) or one could show the normal gavelkind.
As of now a wrong list of gavelkind-heirs is displayed, which would not be true in any constellation.
"if you want to make sure the succession happens properly"
Thank you for the tip. But actually I don't care so much who inherits - as long as I know who he is.