Yesterday, Microsoft showed us that they're back to pushing out Redstone 2 builds to Windows Insiders on the Fast Ring, after taking a 1-week hiatus. 14915 dropped for both PC and mobile. There were some known issues for PC but none for mobile.
Since the build released, Windows 10 Mobile users have been complaining of an issue closing the Action Center. Previously, it could be closed by swiping up in the black area; however, this build killed that functionality. Instead, users have to swipe right from the bottom of the screen in order to close it.
Users experiencing the issue took to Reddit to voice their complaints, in which one of Microsoft's community champs, Jen Gentleman, responded. Gentleman thanked users for reporting the issue and stated that Microsoft is looking into the issue:
Thanks for reporting (and sharing the links ?) - looking into it
It is likely this issue will be fixed in the near future through another build. In the meantime, swiping right from the bottom all the way up will close the Action Center.