If you have the need to paste long documents, why don't you just upload the file at that point?
It's basically what I've come to expect from most Google products at this point: half-baked, buggy, confusing, not intuitive.
If you don't have it, you might be in a Google feature flag jail-- this happens frustratingly often, where 99.9% of users have a feature flag enabled but your account just gets stuck with the flag off with no way to resolve it. It's the absolute worst part about Google.