MareLuce, I did a map on PC and see your problem on the phone, yes, no 'Add as a contact' with 'My Maps'.
But I have a short work-around
At the maps of 'My Maps' I have a long press on the POI, but not the label, on that little flag below the label. A new label appears, I tap on it for to open the location page and star the location as one of 'My Places' (tap on the star icon on upper right corner).
Now I find the location on the 'My Places' tabs and can add it to the contacts ...
Hope my description is clear enough