Seems I found the reason, nevertheless maybe it is worth keeping this thread.
I used WPforms and created a form called XYZ (random name to explain) multi columns layout (to have two fields side by side) but the choices I made seems to be deleted for no reason.
So when I recently made changes to the existing form XYZ, WPForms deleted all column choices and kept all other (non-layout) entry and choice fields plus it created A NEW shortcode for the form XYZ, I wasn’t aware of that. So since I didn’t change the shortcode on the page where the form is used, the old form without the changes still was loaded for users. And it still worked. Oddly enough – the form with the name XYZ still show up on the forms page in backend (and as mentioned it kept working).
Now that I realized the shortcode has changed, I went on the page where the form is used, I changed the shortcode to the new one and now the exakt same form but without multi column layout shows on the page. I’ll have to edit the form once again.
Note to myself (and others) Always check to be sure what shortcode is next to your form. 😉