instead using the scalebar I used the scroll-wheel and I was surprised, that the coordinates changed. I tried different settings of the map-window. If I deactivate the window, than no changes happen, but the map was only gray. An activated window showed apparently no effects if I changed the activation-rules: Coordinates were changed every time during I scrolled the wheel. Reproducible: Always
Thanks Albrecht. Confirmed by comparing to the Marble app itself. When I mouse-scroll (vertical wheel) using in Marble the white cross (as in https://marble.kde.org/img/gallery/marble-desktop-satellite-clouds.png) changes position while scrolling and goes back to the center point afterwards. In forms it stays in a new position so on data editing starts and saving the changes saves the new coordinates. Is that what you found too? A side note is also that zooming (changing altitude) in form maps starts editing. This is the case because altitude is part of the data. Maybe we don't want to have that always and in some cases we want to lock altitude and only allow to change coordinates.
Using the scroll wheel shows that the center of enlargment lays on the cursor. Its a well known function, also pan in the window. This is normal. But if you have determined coordinates, it is not allowed to change them !! For this case, I need urgently a switch to stop the above functions. Because there are two fields in design-properties depending to activities, the second offers: "no activation"; "tab,click"; "tab&click" and "tab,click,wheel" I assumed this could deactivate the scrolling. If so, I would say its a bug if it doesn't work.
I was wrong it is no bug. I saw the cross as a marker for the coordinates. A pan in the window changes normally the coordinates. To save them it is necessarry to leave the form without saving. Sorry!