The card says "the first time I see you damage the enemy Nexus this round." If you are able to trigger it multiple times, that does sound like a bug that should be addressed.
Edit: Realizing now that I misunderstood this comment, so I have a bit of egg on my face. Glad to know everything is working as intended, though.
Sejuani behaves as described here; so the first time she is both leveled-up and sees damage dealt to the enemy Nexus, her ability will trigger. That still supports the scenario u/Fast2Bfast described.