Various of the file browser windows can’t be closed manually... you -have- to either close BM3 or import something.
This is on a 12.9” 2017 iPad Pro running the latest IOS11 beta and BM303
Note trying to close by clicking back in the BM3 window let’s you keep using the BM3 interface rather than closing the file browser as you might expect.
There are some 'hidden' buttons for close (upper right corner) just like there's a 'hidden' edit button to edit the locations where the vertical line is above and left of the search field).
iOS11 had a quite major overhaul of the 'document picker' and 'iCloud Drive' so the upcoming 3.0.4 update should be fixing these problems when it arrives.
Ah sweet... has been really annoying while moving over my BM2 stuff and accidentally clicking the wrong button I assumed it was either an IOS11 or 12.9" screen issue.