Again another bug/interaction with Mirage. When you have maximum cards in hand and the mirage is a card with miniaturize, it will give you the mini version instead and discards the mirage.
Miniaturize is the first effect listed isnt it? That is gonna happen first when you play it. Its pretty consistent that the miniaturize-battlecry minions are specifically worded in that order so you know what order things will happen when playing them. Being at maximum hand-size is what prevented you from getting the mini + mirage and end up at max hand size. Had you played one card prior you wouldnt have had an issue.
Im assuming you expected the battlecry to happen first, filling your hand back to full with said mirage, and just losing out on the mini, but that isnt the order the card does things in. No bug, it happened as it ought to have.
Why do you say this is a bug? What actual game rule or card text is being broken?
I can understand why you don’t LIKE it, but that doesn’t make it a bug.
If your hand is full, don’t play a Mirage minion that also adds another card to your hand. Simple as that.