mongoc_bulkwrite_execute only reports the last-used server_id within mongoc_bulkwritereturn_t.res, which is a mongoc_bulkwriteresult_t. Since that struct is not used with an unacknowledged write concern, this means that unacknowledged writes can never report a server.

PHPC previously reported the server in its legacy bulk write API, but may be unable to do so with the new API.

Note: I reported this issue just to record this feedback and highlight the missing functionality. There is no immediate need to move forward with this.