I've been upgrading my multiple accounts today and when I went to the sales chat and write something to the likes of "I have 150k ready for a 3-month upgrade. Please message me if you're selling" it refused to let me chat and told me to make sure I read the rules before entering the community. I believe reading the rules might have fixed it, however none of these account are new, and I have definitely chatted/read the rules before on all of them. I've been gone for a bit so maybe that's the issue? But the error message didn't come up on every account, only some of them. That's why I'm thinking it's a bug.

I don't believe this is a bug. There was an update about two months ago to make it so everyone had to read the rules before chatting to try to cut down on rule breaking. I'm guessing that on some accounts you haven't chatted since then until now.