I've routinely had problems with the civicrm_api3_multisite_domain_create($params) API call because it would result in a MySQL deadlock error that I traced to the _civicrm_load_navigation() call. When this occurs, the link between the domain_group_id and domain_id is lost because the `civicrm_api3('Setting', 'create', ...)` call occurs only after _civicrm_load_navigation(), and outside of the DB transaction. To resolve this, I've moved the `civicrm_api3('Setting', 'create', ...)` call into the transaction, and the _civicrm_load_navigation() outside of the transaction. See https://lab.civicrm.org/extensions/multisite/-/merge_requests/10 for a merge request to resolve this. (And since the `_civicrm_load_navigation()` seems prone to deadlock in this scenario, I'll separately open an issue to suggest an optional `skip_nav` parameter which bypasses `_civicrm_load_navigation()`.)