I don't think this is really a bug because of how it happened, but it maybe needs some clarification in the FAQ or possibly on the card. Maybe it's too much of an edge case to matter.
Game number 159955806, which included:
- River Shrine: "Trash up to 2 cards from your hand. At the start of Clean-up, if you didn't gain any cards in your Buy phase this turn, gain a card costing up to $4."
- Abundance: "The next time you gain an Action card: +1 Buy and +$3."
I played River Shrine, and played a couple of Abundance cards. I did not gain any cards in my Buy phase, which triggered River Shrine to allow me to gain an action card. I did, and that triggered both Abundance cards which were in play, so I got +$3 and +Buy twice. However, my turn ended instantly (because I was in Cleanup) so I could not use that benefit at all.
Should Abundance even be able to trigger in the Cleanup phase? It wastes its effect completely.
Or, if it does trigger, should that return you to the Buy phase?
I think the card text is pretty clear here. You gained a card, you got the +3 coin and +1 buy, and because it was the cleanup phase, you didn't get to use it.
There are many many footguns in Dominion where you can do something that doesn't quite work out the way you want it to.
Yes, that was unfortunate series of events that lead to the "waste" of the Abundace effect for you. Abundance will trigger on the very next gain of an Action card, which could also be on your opponent's turn (like with a card giving you a Ruin).