I was summoning Neos Knight using using Neos and Aqua Dolphin, but Neos did not gain any attack from at all from it. According to the card, it should gain "half the attack of the warrior monster (not named Neos) used to summon it". So in this case, it should've gained 300 attack but it didn't.
Am I missing something here, or is this a potential bug with the card?
No, original comment was correct. If you read the card text on Neos Fusion it also says it Special Summons the monster "ignoring its Summoning conditions." Thus the reason you can use it on things like Aqua Neos (normally contact fusion only) or Neos Knight.
Additionally i had the same problem with another neos maybe the game mistakes them as elemtal hero neos somehow so it doesn't gain atk
Your answer doesn't make sense because the card states it can only be fusion summoned so neos fusion should count as one since you are able to summon it with it
Did you use Neos Fusion to summon it? Neos Knight's effect only activates if it is fusion summoned - Neos Fusion counts as a special summon rather than a fusion summon, so won't activate Neos Knight's attack boost.