So, this has now happened for the 2nd time in the exact same way. I still have rewards locked of course (3 at the time of this writing). And there was no other special quest (with a crown towards unlocking a challenge) active.

What happened is that I connected a preplaced tile AND with the same placement completed a normal quest. The normal quest completed OK and the preplaced tile gave a new normal quest but without the crown that denotes that it will unlock a new challenge on completion.

Maybe this is normal (in that not all preplaced tiles will give a quest to unlock a new challenge) but if not then it is a bug!
(This is on the latest beta 0.3.0.1b 21102509)