I don't know when this happened, but in all the List/Libraries the Lookup Field is now displayed in the Form, even though the field is not present in the view or shown in the forms editor.
I added the field to the view, then it's shown twice. Then removing it from the view, just removed the copy.
I removed the app packet and installed it from the official store recently, other than that I didn't do any changes the last few months, so I don't know exactly when this started to occur.
Lookup field is showing when it is actually hidden on list view - as shown on the designer I only have the name field on my list view, but the request ID field (the lookup) is also showing when it shouldn't be: