I had 3 creatures in play and gained 10 life in the turn.
When my Firemane Angel in my graveyard triggered to return it to the battlefield i declined replacing an existing creature.
The angel is then removed from my graveyard instead of remaining there.
It may be intended because that is the way a lot of these types of effects have always worked, but if it is it should definitely mention it on the card somewhere
my 2 cents is that the graveyard should act the same as the hand.
if you are prompted to play a creature from your hand and you decline, it remains in your hand.
if you are prompted to play a creature from your graveyard and you decline, it should remain in your graveyard.
I think some spells work the same way from hand. If you decline they are gone. Don’t remember the specific spell, but I realize this annoying behavior sometime.
I saw it happens in cards with more than one effect. You are asked for the first effect, you say no and the second one resolves. One of the cards is infernal reckoning, but I saw it happen in another one, but not remember which one.