This has happened to me too. I used Elemental Hero Neos and Gate Guardian for Neos Knight, and I intended on using de-fusion to split them up when the player I was against used enemy controller. De-fusion simply sent Neos Knight to the extra deck, without bringing any of them back from the graveyard. It definitely seems bugged.
All materials need to be in the graveyard and they need be able to be special summoned from there, if one of those materials aren't in the graveyard or cannot be special summoned, so none is special summoned. In Emma's case, Gate Guardian cannot be special summoned unless it was special summoned firt by its own method and was sent from the field to the Graveyard.
This is not a bug. You need the fusion MATERIALS used for summoning THAT EXACT fusion monster are in the graveyard, you can special summon. This will never work with a Neos fusion monster, because you do not use the monsters as materials.
That isn't why De-Fusion won't usually Summon anything when used on Brave Neos. (De-Fusion only cares about the cards you used as material, not if they're actually named on the card.) However, Neos Fusion doesn't Fusion Summon a monster, so there are no applicable monsters to Summon.
You could De-Fusion one Summoned with, say, Polymerization or Future Fusion and Summon the materials just fine.