AFAICT `document_open_full()` does not set the _per file_ line wrapping, it uses the default wrapping setting because it is generally used to open new files, and they get the default setting as I explained above.
Then `open_session_file()` sets the wrapping to the value saved in the session and tells Scintilla to do that. Your suggested change (as I understand it) will not tell Scintilla, so the display won't be changed to match to the value saved in the session file.
Changing wrappings should not cause debug messages, and indeed does not do so here, so the thing to do is to find whats different about your setup that causes the problem. Do you get a message when you do `Menu->Document->Line Wrapping` which calls `editor_set_line_wrapping()`?