@Anonymous
So, to make sure I wasn't going crazy, I performed the same experiment. I did not see the problem you are experiencing.
Test 1
A Flexible Height Gallery with the Items property set to [1,2,3,4,5,6,1,2,7,8,2,6,7,8,3,4,5,6,2,2,1,1]
A ComboBox with an Items property of [1,2,3,4,5,6,7,8]
A Label in the Gallery template and set the Text to ThisItem.Value and the Visible property to ThisItem.Value in Combobox1.SelectedItems.Value
Performed steps you mentioned as not working, but worked just fine for me.
Test 2
A SharePoint List with only a Title Column. Created a random series of items in the list with the Title set to a value between 1 and 8 (just like you had - Called the SharePoint list Jonas in your honor 😊)
A Flexible Height Gallery with Items property set to Jonas datasource
A ComboBox with an Items property of [1,2,3,4,5,6,7,8]
A Label in the Gallery Template with the Text property set to ThisItem.Title and the Visible property set to ThisItem.Title in Combobox1.SelectedItems.Value
Performed steps you mentioned as not working, but worked just fine for me.
Are you doing something different that what I outlined above?