All the lists and libraries in our classic site collections have reverted to classic view with no option available to switch to modern. It happened all of a sudden and all at once. I've checked SP Admin portal, it is set for modern so that it will be the default. The lists and libraries are set to use the default. The only way I am able to display the modern UI is by forcing it in the list's/library's settings. I've closed and reopened my browser multiple times. I've tried Edge, Chrome and Firefox: the same experience for all. I can't be going to every list and library in the all my site collections to set them manually!
Anybody know what happened? Anyone have a solution? Anyone know where else to check for some sort of tenant-level global setting I might have inadvertently changed that isn't in the SP Admin portal?