Try setting the compiler directive runtimeencoding(ansi) on in the COBOL tab of the project properties. This should tell it to treat PIC X data using the ANSI codeset.
You could also try using CBL_STRING_CONVERT to convert between ansi and Unicode and then use the converted string in your Textbox.
Example
call "CBL_STRING_CONVERT" using by reference test-record
by value 10
by value 3
by reference mypicn
by reference 20
by value 1
by value 0
by reference reserved
returning status-code.
set txtBox1::Text to mypicn