According to my test, it worked fine. So it isn't the issue of the custom visual. Maybe something else. Please refer to this document for details. These two items below are the possible cause that I quote from that document.
1. E-mail subscriptions are not yet supported on reports pages created by the Power BI Desktop live connect to service feature.\
2. Subscriptions may fail on dashboards or reports with extremely large images due to email size limits.
Community Support Team _ Dale If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.