tcl.h has:

#if TCL_UTF_MAX > 4
    /*
     * unsigned int isn't 100% accurate as it should be a strict 4-byte value
     * (perhaps wchar_t). 64-bit systems may have troubles. The size of this
     * value must be reflected correctly in regcustom.h and
     * in tclEncoding.c.
     * XXX: Tcl is currently UCS-2 and planning UTF-16 for the Unicode
     * XXX: string rep that Tcl_UniChar represents.  Changing the size
     * XXX: of Tcl_UniChar is supported.
     */
typedef unsigned int Tcl_UniChar;
#else
typedef unsigned short Tcl_UniChar;
#endif

(See in context here:

That means, with TCL_UTF_MAX 4 Tcl_UniChar is still an unsigned short. This is obviously wrong, because a valid 4 byte utf-8 char is beyond the BMP, that means bigger than an unsigned short.

That define should read as

#if TCL_UTF_MAX > 3

as before check-in

Same in regcustom.h: