I'm a bit startled because whenever I write a comment, I only have a preview button.

I am quite fine with that because I like to double-check my writings anyway, and I am sure that I have never checked that option in my settings.

Maybe the issue is related to some "session" or cookie lifetime? Or you can check the button, but not uncheck it because the software doesn't distinguish between "option off" and "option not set" (BTST)?