If multiple ISM components are set to render to the same RVT and they each have the same mesh/material setup, then intermittently some instances will be dropped from rendering to RVT pages.
Different instances will be drawn to different pages, so that instances will appear to fade in and out as we move the camera.
Load the attached RVTProblem project in the editor.
Observe that many numbered instances go missing as we move the camera around.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-193234 in the post.