With chemotherapy, although common, complete hair loss is not necessary. There may be thinning, partial loss, may not be at all. Therefore, in theory, Walter White's version is quite acceptable. Plus, if I remember correctly, his hair didn't fall out. They began to fall out and then he shaved his head himself. Which confirms that the full dropout was not shown there.
Because this is a movie.
Chemotherapy slows down or stops cell division in the body. Since cancer cells divide very quickly, chemotherapy has the strongest effect on them, destroying all cells that are trying to divide. But in addition to cancer cells, hair cells in the hair follicles also divide very quickly, so the hair falls out. Everywhere. Even eyelashes. And a beard, and eyebrows, and on hands