I did some more tests looking into the MusicXML files.
Both BIAB and Songmaster PRO are using MusicXML 3.0
But they seem to handle the chord ”qualifier/extenstion specification" differently!

Looking in the MusicXML files for some sample chords as an example.

SongMaster PRO exports a chord E7 resp E9 as:

<root>
<root-step>E</root-step>
</root>
<kind>dominant</kind>

<root>
<root-step>B</root-step>
</root>
<kind>dominant-ninth</kind>

And BIAB exports a chord E7 resp E9 as:

<root>
<root-step>E</root-step>
</root>
<kind text="7">dominant</kind>
</harmony>

<root>
<root-step>E</root-step>
</root>
<kind text="9">dominant-ninth</kind>

(strange that it should differ if they using the same the standard).

MuseScore uses MusicXML 4.0 and seems to import both versions correctly as far as I can see.
as well as GuitarPro 8 uses MusicXML 2.0 and seems to import both versions correctly as far as I can see.

But BIAB misses the chord ”qualifier/extension specification” when importing MusicXML from SongMaster Pro ie an E7 or E9 chord comes in as E and similar for others. But the basic chord structure for the song seems to be imported ok (ie chords without qualifiers/extensions). Also BIAB seems to be able to import its own created MusicXML correctly.

Maybe there is a configuration issue somewhere (or some simple change one can modify in the MusicXML file to get it working?)

I don't know if there is one correct way to implement the standard or if both ways are equally correct. But interestingly other software companies seem to be able to handle it so it works for both cases.