This issue has been a bane of my Yahoo email for years now. I was hoping that this latest update would fix it. But alas, it hasn't. I set my preview pane choice to "preview on the right". But everytime I close out of yahoo mail and come back in, the setting is changed to "no preview" and I have to change it. I'm really getting tired of it.
Windows 7, 64 bit
Chrome browser 30.0.1599.69
Yahoo mail long time premium user
By premium user do you mean the paid version? Yahoo plus?
I have tried different things to recreate your problem, and I can't. When I log back in, the preview pane is still there. The preview pane option is not offered if I switch back to basic. It is only available in the full featured version.
Have you tried a different browser? I would start there. I prefer Firefox personally.