Custom visuals only exist in the report that you have imported them to, as described here:
"Important:
A custom visual template is added to a specific report when imported. If you'd like to use the visual template in another report, you need to import it into that report as well. When a report with a custom visual is saved using the Save As option, a copy of the custom visual template is saved with the new report."
https://powerbi.microsoft.com/en-us/documentation/powerbi-custom-visuals-use/
Is there any way to make custom visuals permanent across all reports? It's a bit of a pain having to re-import them each time you create a new report.