There is this strange bug that CurrentBankTab returns 0 (i.e. cant detect) when you are at BankTab(2). Did some debug and here is the result:

Basically it detects all the tabs well except tab 2, and it has got nothing to do with the item of the tab because i tried with other items.
Looking into the function...
Simba Code:
function CurrentBankTab: Integer;
begin
Result := -1;
if not BankScreen then
Exit;
for Result := 1 to 9 do
if GetColor(40 + 48 * (Result - 1), 83) = 2896954 then
Exit;
Result := 0;
end;
It's just grabbing a color at the respective tab (when you are at the tab, the color of the tab is slightly lighter) so i dont see why it will work for every tab but not tab 2.