The debug raw allocator do not return the same alignment as malloc. See  _PyMem_DebugRawAlloc:
https://github.com/python/cpython/blob/master/Objects/obmalloc.c#L1873

The line
return p + 2*SST

adds 2 * sizeof(size_t) to the pointer returned by malloc.
On for example x32 malloc returns 16 byte aligned memory but size_t is 4 bytes.
This makes all memory returned by the debug allocators not aligned the what the system assumes on such platforms.