Almost every day at 5 am local time (living in the Netherlands). It happens right after reset.
The bug persists until I either remove all cookies (which also ends the session on my end) or force it to fully reload with CTRL+F5.

I don't have the means to record it, but I can describe it in detail:
1. Contests end while game is active, reward is claimed.
2. Go back to the contest page (using the dropdown menu).
3. Repeat step 2 until the new contest(s) appear.
4. Attempt to do action for contest, like a pachinko roll or fight against a troll.
5. Go back to the contest page.
6. No points have been added to the contest.
7. Repeat step 4-6 until it randomly starts working or I give up.
8. In case I give up, press CTRL+F5 to refresh without cache (also known as a forced refresh).