I had a closer look at ‘Personal Data Updates’ and found a little bug.
If I tick the checkbox of a custom field and submit, it goes through nicely. Since the form shows again I can click submit again and the checkbox of the custom field reappears.
The user can do this repeatedly and the checkbox will show/hide.
Thanks for catching this bug. In this case, it looks like submitting the form multiple times had the potential to erase the custom field data for a user that was not included in the update, so well caught! I’ve applied a fix in the v18 branch, which you can see here, feel free to apply it back to your codebase: