Problem identified

• Create some custom fields for articles (all for same category and fieldset.

• One of type list (or checkboxes) with some values.

• some other custom fields with showon on different values of the list field.

• Create a new article for the category.

• choose a value for the list-field.

• enter some data in the shown field(s).

• change the value of the first list-field.

• enter data in the newly shown field(s).

• save the article.

When you reopen the article and change the value of the list-field again to the old value, the hidden fields are shown again with the data that was entered.
Also when you use JFilter or so, you can filter the article on that data.

Proposed solution

For me, at some point, at last just before saving the article, all entered data in, at that time, hidden fields should be trown away.
Hiding the field(s) regarding the value of an other field means that those fields have no proper meaning at that time and in that context.

Open questions