Form elements can contain both a URI and a calculated value. When an element contains both of these things, there is an edge case where the value for the element can differ between what is shown when viewing the form and what is recorded in the database.
In the case where a calculated value is used to replace a URI in a form element, the calculated value can be shown when viewing the form, but the URI will be recorded as the value in the database that is used from then on (e.g. exporting to a PDF file).
The development team are aware of this bug and hope to fix it shortly.
Until this happens, we recommend not using URIs and calculations in the same form element.