In meiosis 2, if chromosomes are already in a chromosomal form, why do some cells still have chromatins that shorten and thicken during this phase?

Because after mitosis 1 the chromosomes return to their normal chromosomal state After which they go through the PMAT cycle again. The chromatin shorten and condense which occurs during prophase 1, and in prophase 2. Essentially in meiosis, the chromosomes go into prophase 2, metaphase 2, anaphase 2 and telophase 2.