Hi!
Firstly, thank you for purchasing the asset! I took a quick look at the viper's down attack spritesheet and I do not see any mistakes.
The attack animation is meant to showcase the viper coiling and then rapidly stretching itself to attack the adjacent tile. In the case of it facing down, the viper is shown coiling with its mouth open and in the 3rd frame it is shown stretched downwards and its jaws are closed (an FX animation can be added on top in this frame to add more flair).
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