Basically, I'm just unable to choose the version my server is currently running (because host hasn't updated yet)... Other members are able to choose the correct version on their launcher, but this has happened twice.
Yesterday (on laptop), I wanted to play 18w10a because the server was not updated to b yet. As you can see in the first screenshot, there is no option for 10a.
Today (on desktop), I wanted to play 10b, because the server was finally updated to 10b. You can see on the second screenshot that it happened again.
I believe it's because I never got a chance to download and play the respective versions on either computer. Either way, the versions should be available to download and play until host allows for update.
Intended, see MCL-4046.