Do open market operations permanently increase the money supply?
If these T-bills mature and are repaid then you are correct money supply will decrease. In fact that might be desirable. Just because central bank wants to increase money supply in one year that does not mean they will not want to decrease it at some further period.
When the T-bill matures central bank also have option to then buy new T-bill to replace the old one if they want to keep money supply constant.
Lastly, central banks could also retire the T-bill meaning government never needs to repay it and the increase in money supply is now permanent (or more precisely there might be some other ways how it can be decreased but not anymore through the same instrument so if we are talking only about OMO's it would be permanent).   This is possible but given that it is permanent and central banks might want to not just increase money supply but also decrease it in the future they might prefer just buy new bonds as old ones expire until they decide that they want to decrease money supply.