I see this message in the _internal log so I know the action was called, but I don't have any further clues about the issue.
Invoking modular alert action=sendresults_alert for search="Stale AD Users" sid="scheduler_aGtlbGxleUBzb21wby1pbnRsLmNvbQ__search__......." in app="search" type="saved"
The report also has a regular send email action, which did work.
Running on Splunk Cloud