I use x86 beta release builds (and now, also x86 development release builds), which can be considered to have a super set of features compared to the public preview builds. Everything working in the beta builds will eventually be added to public preview builds. Mature features should be working the same way in both.
I've always used the default titles metadata reading option, so song track length has always been shown to the right of each track in the PE for me (track played or not). I didn't know that the PE song track length waits to be shown after a track's metadata is read. Maybe that is only happening with the public preview build. At one point in the past, I asked that showing song track length in the PE be an option. It didn't happen. I don't remember why I asked for that option.

Side Note: There has been on and off issues displaying song track length in various media library views (mostly History), in the past. Every song track added to the media library has it's metadata read.