I believe this dates back to 859bc9b from CDRIVER-769. Therein, _mongoc_write_concern_freeze was changed to always append w to the BSON representation (provided it was not a tag string or "majority").

MONGOC_WRITE_CONCERN_W_ERRORS_IGNORED was deprecated in CDRIVER-628 and is currently documented as a synonym for MONGOC_WRITE_CONCERN_W_UNACKNOWLEDGED. In that case, I think it should serialize as zero in BSON instead of its raw value (-1).