There are two issues here:
1. When the steel fabrication data is added (when you modify a structural element with any of the Steel tab tool) the beam representation is given by the fabrication shape (see more here: https://help.autodesk.com/view/RVT/2020/ENU/?guid=GUID-8381A08F-CBCD-4215-AB36-82648C07B8F6)
This "transformation" has some limitations (in the same link there are more information). Most important in your care are:
- Start and End Extensions - are replaced by the Shortening modifier.
- The Start and End Join Cutback geometrical position parameters will be ignored and will no longer be available in the Properties palette.
So, the cut you see is a steel modifier that was created from the Start/End Extension. In your case, before adding the steel fabrication (before creating the connection), the Start/End Extension was ignored due the join tool, and the cut value was given by the Start/End Cut back.
Workaround - switch on fine detail level, enable all steel connections sub-categories (just to be sure), go with the mouse over the beam end and TAB until the Shortening is displayed (see attached video). Select it and delete it or change its value.
2. Those two parallel dashed lines... that is the hidden lines representing the web of the element. When the steel fabrication data is added and the beam is cut by a steel modifier, those lines are not cut. This is a known issue - the workaround is to shorten the element by dragging the start/end point, as I saw you did in the recording.
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