Thanks for reporting this issue, and I'm sorry for not answering your email, but there is nothing I can currently do to fix this.

YouTube rolls out these experimental UI or code changes to a very small number of users all the time, and some of them will eventually become new UI versions rolled out to everybody, and some will be ditched in a month or so.

I still have the old playlist UI version, and when I get the new one, I'll add a fix for the Playlist Duration feature.