I bought a new 22.5" OTG last month and I have the same problem with my charcoal grate. My grate is the correct size but after taking a closer look, it turns out that my grate is not quite round so it can fall off the tabs unless I set it down on the tabs perfectly and never accidentally move it (which is kinda impossible when I am pushing the coals around or cleaning the cooking grate with the brush). My grate is also not true so it wobbles too. I bent the tabs which seemed to keep my grate in place most of the time and minimized the wobbling. There are still some positions where the grate can fall.
I contacted Weber and told them about my grate. They nicely offered to send me a new charcoal grate free of charge. I received it super fast (3 days!) and unfortunately the new one had the same problem. So now I just try to be more careful when I push the coals around or when I am cleaning the cooking grate.