While running my own build of Godot 4.5.1 I was seeing heap corruption in D3D12 code. To investigate I made a dev build and found I was hitting a dev assert in the D3D12 CPU descriptor heap management logic:

I believe the problem due to the order of deletion and update, specifically:

free_blocks_by_offset.erase(next->value.global_offset);

next->value.global_offset always matches the key of the pair in this RBMap, so this is erasing "next" itself. So the next iterator now points to deleted memory, but then next is updated and put back into free_blocks_by_offset. Since next points to deleted memory, this is undefined behavior. In my debug builds, you can see in the screenshot that some kind of sentinel values get set to make this "use-after-free" issue more obvious.

To fix this I create a block separate from the one that next points to so I can mutate it before deleting the old block and inserting in the new merged block. I don't know if this fixes the original heap corruption but it does fix the DEV_ASSERT (on line 497 in my screenshot) and corrupted block data.