I activated Miracle Fusion from my hand and chose to summon Elemental Hero Neos Knight. I then chose King of the Swamp and Gate Guardian as the fusion material monsters from my graveyard. However when Neos Knight appeared on my field it had only gained 250 attack points instead of 1875. This means it used King of the Swamp's attack in determining the bonus Neos Knight would receive instead of Gate Guardian's. Neo's knight clearly states that it gains the attack bonus from the Warrior type monster used to summon it, King of the Swamp is Aqua. Somehow instead of registering King of the Swamp as Elemental Hero Neos it registered it as a normal Warrior type monster and Gate Guardian as Neos.
Just tested this out and it worked as it should.

Can you please post a replay of this issue occurring?

Sorry, I don't have recording software. Bug happens around turn 26 or 27.
There does seem to be an error in these cards interaction. But only in certain situations.

Thank you for your report.

Hopefully have this fixed soon.

Next time you can upload replays directly to your forum post by ticking the box next to "Attach files to this post?" when you are making a post.