IOS - Storyboard: Buttons cannot be changed after Xcode update (6.1 to newer)

I have the following problem:

If I load a (functional) project created in Xcode 6.1 or earlier into later versions of Xcode and run the project everything works correctly ... until the first time I edit the storyboard . After that, some buttons stop working (no longer pressed).

If I compare the Main_iPhone.storyboard xml file before and after editing, there are only two differences:

1) Identifying the version of the tools - as expected.

2) The attribute ambiguous="YES"

disappears from the parent view of the affected buttons.

I'm not sure what this attribute is for, and most importantly, I can't get Xcode to leave the attribute where it is, or fix the buttons to work without it.

Any ideas?