I logged this with support and received the following response:
“Defect Workaround: place a “Text field” over the field “TAX” and make sure the background color of that new field is the same as the background of your form”
The status of this is: “This issue will be considered for a future release.”
There is however a much better (at least in my opinion) workaround which involves renaming the alias and display name of the Tax field in DB Admin to anything that doesn’t include a % in the name.
Now you can edit it as much as you like.