When entering some Unicode characters in Books and Signs using Alt+code, they dont display the correct character according to the convention list (CP437 and others)
Examples:
Alt 236 should give ∞, gives ý instead (Alt 0253)
Alt 239 should give ∩, gives ´ instead (Alt 0180)
Alt 251 should give √, gives ¹ instead (Alt 0185)
All this symbols can be correctly copy-pasted into Books and Signs
Alt codes depend on locale set in your system. It seems OEM codepage (one used for codes not prefixed with 0) in your case is set to CP 850 or similar, instead of expected 437. Minecraft does not perform any extra modifications.