if you play medium then common is more common as the name implies

.it might not be in the reward list but it is included I think

I wouldn't care if I got common on top of medium. Medium is shown as a reward and as such should be guaranteed. Plus I've always gotten medium thread out of it up until now.

But medium thread isn't even guaranteed on HARD. (about 80%-90% I think?)
In fact, there is a slight chance you won't even get common thread on hard.

Just because it's listed doesn't mean it's a guarantee, it is a list of POTENTIAL rewards.

Then it should say POTENTIAL rewards, not Rewards.

I've never failed to get it before so it's new to me.