I've found that sometimes Kontact's main toolbar is messed up and ordered backwards (please check attached images).
Other times buttons added by me just disappears when after quiting and launching Kontact again - albeit this just happened twice 'in a heck of long time'.
Steps to Reproduce:
1. Accommodate the toolbars as shown in the attached pictures.
2. Add or remove buttons to any other toolbar than the main one.
Sometimes, too often, the main toolbar buttons order will be reversed.
Nothing more than the toolbar that I'm customizing is changed.
Created attachment 81030 [details]
Kontact main toolbar showing ok.
Created attachment 81031 [details]
Kontact main toolbar showing messed up after editing Navigator toolbar
Will be fixed with KDE Frameworks 5.24.
*** This bug has been marked as a duplicate of bug 64754 ***