Strange blocking on readable secondary after reboot
It sounds like you experienced the behavior that's described in this post on the PFE blog:

AlwaysOn Availability Groups unable to query against readable secondary replica database: Wait Type HADR_DATABASE_WAIT_FOR_TRANSITION_TO_VERSIONING

Essentially, there happened to be a long-running transaction on the primary when the secondary was made readable, and thus queries will be blocked in the secondary until the snapshot-related row versions are available.  I imagine removing and re-adding the database was coincidental with the long-running transaction finally completing.

So this behavior, as described in that blog post, is by design.

However, if there was not a long-running transaction, then this could be a bug.  There is a comment on that blog that indicates others have had this problem:


  I am facing the issue after Secondary SQL Server reboot after OS patching. Do not see any open transactions in primary prior to the reboot. There are two databases in the AG having this issue. And we have been waiting for more than 15 hours but still readable replica is not able to process any select query for those databases.


If you are able to repeat the behavior, it would be good to report it on the feedback site and / or engage Microsoft support if you have a support agreement.