Stepping through the JS reveals that when executing the function fieldProperties the evt value passed in does not contain definitions for attributes srcElement or target (both undefined). It's as though these don't exist or have become corrupted somehow.
This only appears to happen on a specific form, other forms work ok (evt has both srcElement and target).
I have exported and imported the form and the same thing happens and we have not edited any of the JS related to XForms.
Has anyone else encountered this and know what causes the issue and how it might be resolved?