Hi,
Yes, we were able to figure out that it was indeed a bug in the movie render queue, causing it to add RenderTargets to the gc roots and never removing them. The bug was in the engine from 5.0 to 5.2 and is fixed from 5.3 onwards.
However, if a bug leaking ~100mb ram per call goes unnoticed for 3 versions of the
engine, it should be clear that this use case has not a high development/testing priority for most of the engine developers and users. So be prepared to figure things out yourself, and be aware that your mileage may vary.
Hope that helps