Mixer displaying incorrect instrument sound selections for non-default soundfonts

There is a bug in the "Sound" drop-down in the Mixer dialog when a soundfont other than the MuseScore default has been added (where the soundfont has not been saved to the score). On adding the new soundfont, rather than showing the actual instrument sound as being selected, it shows the first instrument in the soundfont's instrument list as selected.

Steps to reproduce are as follows...
1. Create new score
2. Skip past the score information page
3. Choose instruments
4. Add flute, hit Finish
5. Open the Synthesiser
6. Add non-default installed Fluid soundfont (e.g. Sonatina_Symphonic_Orchestra.sf2)
7. Delete default Fluid soundfont (MuseScore_General_HQ.sf3)
8. Add some notes to first measure (observe that the notes sound as a flute)
9. Open Mixer and view the Sound drop-down...

Expected result : Sound drop-down for flute shows the equivalent Sonatina instrument (Flute Solo) as selected
Actual result : Sound drop-down for flute shows the first Sonatina instrument (Steinway Grand Piano) as selected

The same system behaviour occurs in other similar scenarios. For example, when starting up MuseScore, then re-opening a score which was saved with instrument sounds from a non-standard soundfont, and then adding that soundfont in the Synthesiser, all such instrument sounds now show in the Mixer as the first instrument in the soundfont, rather than the actual sound selected. In these conditions the Mixer display does not reflect the actual sounds selected (and heard), which can be very confusing for the user (especially a novice user who may now believe they have incorrectly selected instrument sounds).

OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 3.4.2.9788, revision: 148e43f