I have ruled out the use of PDF processors other than Adobe Acrobat or Reader because this has actually proved to seriously corrupt our forms.
From additional information I gathered from the user who experiences this weird form behavior, I now understand that:
1. the issue is not wide spread and it may actually user specific
2. however, his computer+Windows+PDF processor do not seem to be the issue: Lenovo X1 Tablet / Win 10 Enterprise 2016 / Adobe Acrobat X
3. The form behavior issue is not limited to one field but seams to appear on any multi-line text field, of which I don't think any includes custom script
4. It seems to happen after clipboard paste operations (especially from Word), but it’s not a pinpoint fact, definitely doesn’t happen all the time
I am now trying to figure out when this started happening, whether it is something introduced by a specific version of the form.
I must say it is a pretty complex form: many pages long, lots of scripts so I am reluctant rebuild the form from scratch, especially considering the user sees that as an annoyance rather than a true impediment.
Just to make sure, sometimes any other software than Adobe Reader may cause bugs like these when using xfa PDF forms. Ensure that the user is using the latest version of Adobe Reader or any version above the minimal mentioned version for the PDF form.
I've already ran through real weird stuff coming from users only because they use another software to read PDF.