Been having this same issue and after spending all morning researching and testing, it appears that this is Player Experience issue on certain client apps.
I’m able to get this to work properly on Web Browser but not on my Roku client app, where it attempts to play the preroll and buffers to 99% but never plays.
A Plex Employee responded on this post in March PreRolls not working
“It will be fixed among other Player Experience settings.”
Now when that is expected or if that is still happening, I’m unsure.
