Has anyone experienced this? I've already converted some Visual Force Pages to Lightning Experience thru enabling them in settings and adding lightningStylesheets="true", it already appears to Lightning Experience with the lightning feels but it just suddenly turns back to classic feels and after some time it just reverts back to lightning feels. I'm not sure if what I did with my code that may have triggered it. I hope to hear some feedback from those who have experienced this. I'd highly appreciate it. Thanks!
When you open the VF page with lightningStylesheets = True from a classic user i.e when you are viewing salesforce in classic globally, it shows in classic feel.
And when you view the VF page as Lightning User i.e., when salesforce is viewed in Lightning globally, it shows in lightning feel.
I've observed the above behavior in my experience.