Beirand doesn't watch Roggvir's execution.
Tested on a 100% vanilla profile with and without USSEP installed. Easily replicable if you want to test it. Just use the command "coc solitudeorigin" and observe Beirand's behavior.
Vanilla: Beirand and some NPCs who are going to watch the execution stand still for a few seconds, and then the scene starts and they walk to the location to watch.
USSEP: The NPCs move to go watch the execution, but Beirand instead goes towards his forge. Sometimes when entering Solitude he is already working on the forge. You can even hear him say "Traitor!" if you get close to him.
Initially I thought it might be related to
Bug #15452, since this package remains active, but reverting the changes has no effect, so the problem is probably in some other modification.