This seems to be a weird Heisenbug that’s just disappeared for me, for no obvious reason. I’m seeing a different error now in files making heavy use of Jane Street PPX rewriters:
Merlin failed with error: "Error while running external preprocessor
Command line: ../_build/default/.ppx/ppx_jane/ppx.exe --as-ppx --cookie 'library-name=conveyance' '/var/folders/px/knyrxd_d3wb4220y4j7cmkg40000gn/T/camlppx8a5508' '/var/folders/px/knyrxd_d3wb4220y4j7cmkg40000gn/T/camlppxf8d2f4' 1>/dev/null 2>/dev/null"