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This is the expected behavior for precast beams.
They behave like steel or wood beams in that when they join to other elements, they have a cutback, not a merge.
If you need the beam to extend and merge, you should use a “Concrete” beam type.
Note that this behavior is not controlled by the material of the beam. You can assign a material of “Precast Concrete” and still have the join behavior.
To control the join behavior, you need to edit the beam family and change the “Material for Model Behavior” parameter.
Joe