[xetex-reference 2018-12-07]
The description of \XeTeXcharclass starts as follows
Assigns a class corresponding to ⟨interchar class⟩ (range 0–255) to
a ⟨char slot⟩. [...]
However, the range has been extended in XeTeX version
3.14159265-2.6-0.99994; it should now rather be
Assigns a class corresponding to ⟨interchar class⟩ (range 0–4095) to
a ⟨char slot⟩. [...]
Similarly, the text in the paragraph should change 255 to 4095 and 256
to 4096.
Werner
