Encounter a problem that print wide_character in C

The terminal spacing between characters is not the same horizontally as it is vertically, meaning each character fits into a rectangle on the screen and not a square. Given your shape is a square □ it cannot occupy its whole rectangular space and leaves blank space on the top and bottom.

In this image you can visually see the height and width of each cell compared the square character.

To fix your snake game problem you could try add spaces between each of the horizontal squares or try different character combinations like []

I find the height of the cell is roughly twice its width so two characters usually make a square

(no spaces)
□
□□
□□□
(with spaces)
□
□ □
□ □ □
(other characters)
[]
[][]
[][][]