Mintty seems to be interpreting Alt+nnnn codes in OCTAL instead of decimal, based on the Unicode code point and not the Windows Alt+nnnn codes. For example, Windows' Character Map applet shows the "Euro" symbol as Unicode character U+20AC and Alt+0128. Entering Alt+0128 in mintty has no discernible effect, but if you convert hex 20AC to octal and enter that (with a leading zero to indicate that it's octal, Alt+020254) you DO get a Euro symbol.
It also appears that without the leading zero the number is interpreted as decimal, again selecting the Unicode code-point. This behavior is what I'd expect from a POSIX-compliant app.
