This function, when merging batch indices, uses local TArray declared as
TArray<TPair<int32, int32>, TInlineAllocator<100, TMemStackAllocator<>>>
However, if the secondary allocator (TMemStackAllocator) is used the code stops at checkSlow inside FMemStackBase::Alloc().
Reason: Stack allocation requires user to use FMemMark first (as it happens on many places in UE4 code)
Fix: Create FMemMark before TArray in the code.