According to posix standard for printf:
If the leading character is a single-quote or double-quote, the value shall be the numeric value in the underlying codeset of the character following the single-quote or double-quote.

This implies that it should be the character's codepoint, which is what is used in coreutils and bash.

In busybox, instead it can return the value of the first of byte of the character.

Examples:
* 바
   HEX codepoint: BC14
   DEC codepoint: 48148
   Hex UTF-8 bytes: EB B0 94
   (UTF-8 bytes converted to DEC): 235 176 148
https://www.cogsci.ed.ac.uk/~richard/utf-8.cgi?input=%EB%B0%94&mode=char
* 학
   HEX codepoint: D559
   DEC codepoint: 54617
   Hex UTF-8 bytes: ED 95 99
   (UTF-8 bytes converted to DEC): 237 149 153
https://www.cogsci.ed.ac.uk/~richard/utf-8.cgi?input=%ED%95%99&mode=char


# busybox printf '%X' "'바"
EB
busybox printf '%X' "'학"
ED
# busybox printf '%d' "'바"
235
# busybox printf '%d' "'학"
237
(these are the HEX and DEC values of the first byte of the character)


Then the printf from coreutils/bash
# printf '%X' "'바"
BC14
# printf '%X' "'학"
D559
# printf '%d' "'바"
48148
# printf '%d' "'학"
54617
(which are the HEX and DEC values of the character's codepoint)

Same happens with multibyte Chinese characters.
# (coreutils) printf '%X' "'传"
4F20
# busybox printf '%X' "'传"
E4

传 has HEX codepoint 4F20 and Hex UTF-8 bytes: E4 BC A0