First, considering we are still in beta, it is likely (thought not common) that updates will break things.
Anyway, the job of keeping things in sync is on HexOS. If HexOS tells you you can update TrueNAS, they are ready for it, but it is not mandatory.
(it could happen that, eventually, HexOS will tell you that you must update TrueNAS to be able to receive new versions of HexOS, but we are not there yet)