It could be this GC issue 15 as described 9 by Instagram. It looks like ~this is fixed~ there is gc.freeze() in Python 3.7, ~so what is your version?~ that can be used before fork to benefit from COW semantics.
You can try disabling gc and see if you see the same work-around. Disabling GC has other issues, specially with cyclic references though.
I think think it is safer/easier to use mmap or numpy.memmap 28 if it is an array.