Tech issues. I called yesterday because I had the exact same problem you described and the exact same worry about buying something I possibly couldn’t use.
Call centre agent thanked me for bringing the bug to his attention and confirmed that the flights were available. You can call in, too, if it’ll make you feel better, but a quick search on ExpertFlyer indicates that the four direct, nonstop flights on both your departure and return dates have ample M space available.
Edit: looks like @
yyz_atc_qq beat me to the EF results ^