A clarification...
The quests that gave 50 ribbons were quests 107-109 out of 130. There was apparently a bug that was operative for a couple of hours (which I don't believe has been officially acknowledged, even over a day later) where the quests were partly driven by the rules of a previous event, where there were two quest lines (thus 2 offered quests at a time) and a longer (and thus lower) quest payoff, that probably explains the drop to 40-ribbon payoffs.
However, that apparent bug was apparently fixed. That you are now seeing quests that pay out only 20 ribbons is probably because you have finished the first 130 regular quests of this event, and have entered the bonus quest stage, where the type of quests are harder and the payoff will remain fixed at 20 ribbons per quest.