Database restore using SSMS leaves source database in restoring state
This is "normal" behaviour in SSMS if you check the box "Take tail-log backup before restore" and the box "Leave source database in the restoring state"

It may not be intuitive and it's probably not the best decision ever to check those boxes by default but you have to un-check the box "leave source database in restoring state" to avoid that behaviour:



So it's not a bug, it's standard SSMS behaviour that changed in a more recent version (I believe those options appeared in 2012 but I'm not sure).

If you want to avoid strange or buggy behaviour in SSMS your best option would be to perform your restores using T-SQL scripts so you have something reproducible even when you change your SSMS version.

See this connect item which clearly states this is by design.