Melffy Wallaby has the wrong effect code. Melffy Wallaby summons any two Level 2 beasts, when it’s just supposed to be Melffys.
Bug reproduction steps
Play Melffy Wallaby. When your opponent summons a monster, activate its effect. You will be prompted to chose two cards to special summon, but it’s not restricted to only Melffys.
Screenshot OR error code
Only Mellfys should be showing up in the special summon selection.
What OS are you using