@dadreamer, nice find!
Apparently these private properties should do exactly what you want: control SH behavior, but they allow to only control the size of large blocks (which shouldn't be the root of this problem) and don't seem to make any difference in my test anyway.
Library replacement, on the other hand, eliminates this problem completely, even for blocks <256 bytes.
Edit: Selective Deallocation is also available as an INI key along with a key "overanxiousMemoryDeallocation" (which doesn't help either).