I think chances are that this assertion issue might be somehow related to a previous assertion issue . I have some rough indicators for this guess. First, the previous assertion issue has been fixed in 2017.2.0p2. With this very same version, the new issue described in this thread started to appear. Second, based on my test results, I think that both assertion issues are related to scene complexity so that a higher scene complexity increases chances that the issue appears. Also, they both seem to be somehow related to the scene hierarchy.
So, in order to find the root cause of this assertion, maybe it is also helpful to look into the repro-cases and solution of the previous issue.