The symbol position is not rotated so to find the rotated bounding box you can construct the box around the 'symbol geometry' and then apply the instance transform associated with the family instance. What I mean by 'symbol geometry' is the geometry shared by multiple instances before it is transformed into the instance positions and not the geometry of the family symbol which exists without any placed instances. If a beam (not modified by cuts) has the same length and family symbol as another beam then it likely shares the same symbol geometry. If the family symbol is the same for two unmodified beams but they have different instance lengths then they no longer share the same symbol geometry. If the beam is cut then it becomes a one off and no longer has shared symbol geometry. There is a bit of terminology overlap which confuses the situation.
For the case of a cut beam it still contains a GeometryInstance with an instance transform. So you can still use the inverse of that transform to establish the symbol position for the geometry then construct the bounding box and transform it back to the instance position.
The usefulness of rotating the bounding box depends on what you ultimately want to do with it. Noting that BoundingBoxIntersectsFilter etc. actually takes an Outline object which is similar to BoundingBoxXYZ but without the transform allowing it to be rotated. So BoundingBoxIntersectsFilter will always work on a bounding box parallel to the internal coordinate system.
