Alerif написалBut I can't understand why this problem doesn't show up in other animations.
If texture import settings are the same on your other atlas textures, this could be due to resolution, width or height not being multiple of 4. IIRC ETC2 compresses in blocks of 4x4 pixels (as many compression formats do). So I guess the above texture is a multiple of 4 in both width and height and the others where the problem does not show up are not.
You should however see a message in the texture importer settings that they are not compressed due to their resolution.