High virtual memory usage, reset_decommits doesn't work

Hi! I'm evaluating using mimalloc-secure 2.0.3 for our game instead of Unreal Engine's default FMallocBinned2.
The runtime performance seems good, I have not measured any significant regression from this switch.
However, it seems to not decommit memory nearly as much if the memory usage of the game goes down.

My configuration:
MI_SECURE = 4

The target for comparison, using FMallocBinned2:
- Game started
- Loaded large save
- Cleared large save
- Loaded large save again

The initial results using mimalloc:
- Game started
- Loaded large save
- Cleared large save
- Loaded large save again

That's... not great in comparison.

I enabled the options page_reset, abandoned_page_reset, gives significant improvements for the working set, but no improvement on unnecessary memory being decommitted again.
- Game started
- Loaded large save
- Cleared large save
- Loaded large save again

The option segment_reset seems to not be hooked up to anything.
The option reset_decommits causes the game to crash instantly during static initialization, if page_reset is also enabled (otherwise it does nothing):

Start by locating the handling for page_reset, abandoned_page_reset, segment_reset, and reset_decommits, then reproduce the static-initialization crash with MI_SECURE=4 and page_reset enabled. Compare the decommit behavior after memory usage falls. Done means reset_decommits no longer crashes or silently does nothing, and the reported decommit behavior is covered by validation.

Written by the indexing model from the issue text.