Why were you there? Need to resolve a question in your mind or conscience? Getting or verifying information? You were waiting for someone at the Mass?
If you were in good conscience or wanting information, no problem.
If you decidedly believe the indult is invalid, that Our Lord is NOT present, the priest is invalid, just a layman masquerading, then you should have high tailed it out of there.
I’ve heard more than one priest suggest cradle traditionals to sit in back of a novus ordo just to observe, to compare the True Mass to the parody.
If you were at serious risk to violate your conscience, you probably committed a venial sin.
I’m obviously not an expert in moral theology or a priest!
Ask your priest in your next confession.