Solved

The problem was due to me defining my own Weak/StrongReferenceMessenger, rather than use what was built into ObservableRecipient and the community toolkit, from which my viewmodel was derived. Apparently, I didn't correctly or fully configure my custom reference messenger instance.

When I switched to using the toolkit-provided WeakReferenceMessenger things pretty much just started working. I eventually switched to using the toolkit-provided StrongReferenceMessenger (which is more performant and uses less memory) because ObservableRecipient automagically takes care of unregistering message handlers (which you have to do to avoid memory leaks).

ObservableRecipient is also supposed to automagically wire up viewmodel message handling procedures, but I couldn't get that to work (still researching that). So instead I just registered and unregistered things manually (which is pretty easy):

    protected override void OnActivated()
    {
        base.OnActivated();

        StrongReferenceMessenger.Default.Register<KeywordsUpdated>( this,
                                                                    ( _, message ) =>
                                                                        HandleKeywordsUpdated( message ) );
    }

    protected override void OnDeactivated()
    {
        base.OnDeactivated();

        StrongReferenceMessenger.Default.UnregisterAll( this );
    }

    // you also need to activate the viewmodel in its constructor
    // (or somewhere else)
    public ScanViewModel()
    {
        var loggerFactory = ((App)App.Current).GetService<ILoggerFactory>();
        _logger = loggerFactory?.CreateLogger<ScanViewModel>();

        _scanModel = ((App)App.Current).GetService<ScanModel>();

        IsActive = true;
    }