Rolf,
It sounds to me like you are making changes to a screen that is being used by users at the same time. This is bad news. Why? Well if you're making changes while users are connected to that screen, they're going to get funky results because you're moving around actions, etc.
You should either:
a) make changes while users aren’t using the screen OR
b) make a copy of the screen, when happy with the changes, make that version live to their users.
Best practice from Method is option B.
Making changes to a screen in the designer will affect some parts of it, mainly actions, even before you publish. The publish button means that you want to screen to be re-drawn with the latest objects based on their location and references. This process also ensures that objects being called in actions are properly referenced based on their position on screen. The screen is never in a “hibernate” state meaning if you start moving actions around, etc. it will affect the screen. This is not a new issue, it's always been this way.
You should always make a copy of the screen to avoid users accessing a screen in the middle of being altered.
FYI - some actions may work before you publish them but you'll get mixed results. You should always publish before you test a screens action set out. Why? It needs to generate the pages for the screen itself and it needs to update any relevant actions within calls being made to other fields/objects. Publish is a complex process and needs to completely finish for your page to work correctly.
Moving forward this will become less of an issue as our codebase improves. There are already plans for a new architecture which will lock all published pages so any edits will be on a totally different version to the screen (seamless to the user of course).
Follow our best practice above and you should never run into this problem.
~C