Hello Bojan,
The "Base plate" joint point of insertion is always at the lower point of the beam.
If the beam is perfectly horizontal, then the chosen point is...random, and the joint can't be added on both beam ends.
For your information, we have a feature request (in our "Wish list") to add an option in the "Base plate" joint that allows the user to select the beam end through a combobox, and remove any UCS dependencies or orientation.
Regards
Philippe