in synthv1 UI, on menu Help > Configure... > Options, see if switching "Use desktop environment native dialogs" makes any difference.
on a side note, i'd also recommend you to try the repos, most specially install the Applications (focal) PPA, in your case, and update both qtractor and synthv1-lv2; the later is specially tailored as fully self-contained (using Qt 6.2.2-static atm.) and should cause you no worse or arm (even under Ardour ;))
in synthv1 UI, on menu Help > Configure... > Options, see if switching "Use desktop environment native dialogs" makes any difference.
on a side note, i'd also recommend you to try the repos, most specially install the Applications (focal) PPA, in your case, and update both qtractor and synthv1-lv2; the later is specially tailored as fully self-contained (using Qt 6.2.2-static atm.) and should cause you no worse or arm (even under Ardour ;))
hth.
cheers