Hello,
I then changed themes, which looks to reset all dialogs and re-insert the deleted ones.
This is the default behavior, when changing the theme the custom dialogs are reset because each theme comes with its own recommended default dialogs and a dialog can have specific settings depending the theme. Also, you get a warning when change the theme as it can see in the bellow attachment:

- Changing Theme -warning.png (409.74 KiB) Viewed 3558 times
If you have other questions, please let us know.
Best regards,
Dan