weird behaviour of groff / preconv

[Ulrich Lauther]

Hi,

I process this input 
        .ps 20
        This is a partly \f3bold\fP text.

        This is a partly \f3böld\fP text.

        .ds MyBLD \f3\\$*\fP

        This is a partly \*[MyBLD bold] text.

        This is a partly \*[MyBLD "böld"] text.

        This is a partly \*[MyBLD böld] text.

with 
        groff -Tps -k test > test.ps

For all but the last line I get the expected output.
But the last line gives me
        test:12: missing `]'
        test:12: invalid base glyph `u00F6\fPld]' in composite glyph name
        test:12: warning: can't find special character `'
The output of 
        preconf test
is (I show only the last lines):
        This is a partly \*[MyBLD "b\[u00F6]ld"] text.

        This is a partly \*[MyBLD b\[u00F6]ld] text.

It seems groff expands to b\[u00F6]\fPld instead of b\[u00F6]ld\fP.

Is this behaviour to be expected, or is it a bug in groff? 

Kind regards,

     ulrich