Funny, but others have made similar complaints recently. I don't know why it is happening. The way around it is to import your videos into the Photos app, and then access them from the Photos item in the My Media sidebar of an iMovie project. When you click on the Photos item your photos and videos will be displayed in the browser screen above and to the left of the time line. You can choose to have them display in a number of different categories from the drop down menu at the top of the browser screen. The photos library must be designated as the system library for this to work. So, if is not working, go to your Photos app general preferences and check the box that says "use as System Photo library". If the box is grayed out that means it is already your system library and you need not do anything further.
-- Rich