In LibreCAD 2.1.3 , if you have Arrow size set to 0 in Drawing Preferences, and do have an angular dimension
in the drawing, always this will be selected when you try to select any other single object. This is a bug recently discovered and fixed in the latest 2.2.0 rc version.
The workaround simply is not to set Arrow size (dimensione freccia) to 0, even if you use ticks (size other than 0). Looking at your screenshot it should be all right. Maybe you have changed this setting recently?
I have no other explanation. If the problem persists you may report again.