So usually when objects are on a timeline there are bubbles that let you fade in and fade out. I was editing a project last night, and all the bubbles were there. I saved it and left it open on my computer overnight. I must've turned off an option? Because the other projects all have them. The A and B symbols have disappeared from the thumbnails on timeline setting.
I'm running it on Windows 10, 16gb RAM, 200gb hard drive, Core i7 8gen, Acer Predator Laptop.