SSAS Tabular Row-Level Security: Apply restrictions on dashboards already shared?

I believe you'll want to look into using a Content Pack to distribute the report. Shared reports effectively show data for the original user (source sender) to the shared individual (receiver). Thus, when the recipient views the shared report, they see the original data. When they actual edit the report a new query is created using their EffectiveUser, and thus the data is filtered Report content distributed through a Content Pack will use the viewer's credentials (EffectiveUser) when requesting the data from SSAS.