This is interesting and indeed a bug that should be fixed.
But it seems there is a somewhat complicated way to force Band in a Box to play the root of the key as slash bass. To do this, you have to specify the slash bass in each bar individually via the chord builder, for example D/D. If you only copy the bar into others or make no chord input in the next bars, the slashbass will be suspended again. In my example below I did it like this for the first two bars, then it works with the D in the bass track. But I have not tried this with other styles. It will not show the slash bass in the chord view and notation view.
By the way: the keyboard shortcut ALT+F5 should also get fixed as this still does not invoke the chord settings.

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Biab/RB latest build, German Language Pak, Win 10/64 Pro, 1 TB SSD, 32 GB RAM. Notation SW Finale, StaffPad, SmartsScore – Playing Double Bass, Guitar, Piano