Per https://www.mail-archive.com/users@lists.open-mpi.org/msg31021.html, Nils Moschuering (github ID unknown) is running into a problem with MPI I/O running on more than 1 rank using very large filetypes.

See the linked email thread for much more detail, including a reproducer program.

@edgargabriel is following up. I am simply filing this issue just for tracking purposes.