We have this problem pretty often but it seems to appear randomly.
In the Window-menu when you select a different window FileMaker Pro shows a window one down form the selection. We have video example here: FMP17A window select - YouTube
We start from window and file named 'DataLink'. Then we go to menu and select window named 'key'. Instead of window 'key' opening window named 'Tpide' opens instead.
If we recall correctly this was a problem with former version also. At least on FMP16.