I just finished bulk-marking them as “not spam” in an attempt to undo the work, and I always check my spam inbox manually anyway, but I just thought this was… interesting, as I’ve never had anything remotely like this happen before in my life. Does anyone have any idea of how this happened? Why is such a piece of software still allowed in managers?


While you’re technically correct I feel the need to add that Evolution is an excellent mail client.
I have never had the problem described by OP, but I have never connected Evolution to a Gmail account either.
Here, I suspect a user setup error involving Google always doing things a little differently, breaking specs, just because they can and to piss the rest of us off. And to hold on to their quasi-monopoly of course.
In other words, Evolution devs/maintainers have the choice of doing it properly for most email providers that follow the IMAP/SMTP specs, or doing it properly for Gmail.