Assuming B3:17/2 is not written to elsewhere it will stay on, unless/until the PLC leaves and re-enters run (eg due to a power cycle) with the subroutine switched off.
In that scenario the prescan will turn off the bit, and it will never be scanned to turn it on again until the subroutine is enabled.
In the scenario where the subroutine is enabled but then disabled while running, you are correct that it should stay on.