Most of these class of switches have series Fets at the input junction, and some shunt fets along either of the two output paths. Obviously, when one arm of the switch is set into isolation mode, the shunt fets are biased on (short circuit to ground drain to source).
But what is your point? Your transmitter should not care if it is looking into an open or a short as a load, since either will totally reflect power back to the transmitter. If your transmitter is sensitive to a non-matched load, either an open or a short might blow it up.
Most people are carefull to turn off the RF output on the transmitter BEFORE they switch the T/R output switch from insertion loss to isolation state. This protects the transmitter, AND protects the switch (which often can not handle the full RF power while it is transitioning between states). One way to do this is to put A spst switch before the transmitter input, and activate it with the proper time delays so the output SP2T switch is never transitioning under full RF power.