Unable to Re-arrange Transaction Body Fields Positioning when Customizing a Form

This happens when there are lots of customization applied to the form. To fix this issue, move the target field to the bottom of the list then save the changes.

1. Navigate to Customization > Lists, Records, & Fields > Transaction Body Fields.
2. Click the custom field that can't be moved.
3. In the Display tab, choose Last in the INSERT BEFORE field.
4. Click Save & Apply to Forms.