Look at the source and see if the names have been merged there. If so, correct them. Should be trivial to do so. That should fix the problem, though, if the collapsible is edited in App Designer again it might happen again.
If the names are not merged in the source then it sounds like some sort of bug in Bootstrap itself. But this seems unlikely.
Let me know what you find.
If you are comfortable working with the code, an option would be to work in the code view (not the design view) - especially after you have the general set up of your UI.
App Designer has been deprecated and will be removed, it is no longer being supported. Please see the release notes for details > https://software.intel.com/en-us/xdk/docs/release-notes-information-intel-xdk <