Each episode has a different duration. Consequently, the episode duration is set at the episode level.

The episode duration at the series level should only be used when all the episodes have the same duration. Otherwise, it should be locked empty.

We don't have a field to store the complete series duration.

Thanks. How do we get the correct running time to show up on Letterboxd then?

You need to ask this to the Letterboxd support team.