I've finally found a place to complain about this problem!
Not only am I having this problem on my brand new S9+, but I was also having it the past 2 years on my S6. So it's obviously not the phone and not the Facebook version. Someone suggested I log into another Facebook account on the S9+ and see if the problem persists, but I'm not going to the hassle of that, since someone here already tried it and that solved the problem, which means, again, the problem isn't the phone or the Facebook version. The only possible answer is that it's in the Facebook account settings. But what kind of setting would that be? "Don't give me the ability to post on others' walls when I'm using a Facebook mobile app."