I came across a very relevant mingw bug report:
https://sourceforge.net/p/mingw-w64/bugs/465/
in which they were trying to clear an 8-byte jmp_buf field with a
4-byte instruction. That would go a long way towards explaining the
sort of problems we're seeing ... except it was fixed five years ago.
I trust none of the buildfarm animals are still using old mingw
versions.
regards, tom lane