When the replication lag is non zero ( was several hours in that specific occurrence), we see what looks like gridFS corruption when reading data from secondary nodes:

mongocxx::gridfs_exception: expected file to have 1 chunk(s), but query to chunks collection only returned 0 chunk(s): a GridFS file being operated on was discovered to be corrupted

We don't update documents in gridFS, we only create or delete them.

Let me know if you need any other information.