@Dylan Zhu-MSFT Using the built-in VS problem reporting (Thanx Dylan) I discovered there have been over a 128,000 reported incidents of this bug.
According the incident being worked (1317364) this is supposed to be fixed in version 16.10 which is currently classified as Pending Preview Release. Not being privy to the release roadmap I don't know the target date for pushing out a generally available 16.10.
Until that release is available the only partially viable work-around is to close any "tear-off tab" windows prior to Exiting the solution/VS. That will at least enable re-loading of docs in the main VS window on the next solution load.