There is a report when you have both trustworthy and compliant keys for all recipients and prefer S/MIME enabled the interal resolver of Outlook would still resolve to OpenPGP while the external resolver resolves to S/MIME.

This might be possible I think because the internal resolver could assign directly verified keys a higher trust level then S/MIME keys and would then prefer OpenPGP.

The intend of the customer though seems to be "if at all possible use S/MIME" instead of "prefer S/MIME"