so far I have only had this problem on BT hardmode on sith sorcerers (healers).
After zoning I sometimes see no visual indication nor indication in my buf bar that he is guarded and I can't cast it on him again (get message target is already guarded).
I can guard other party members at that time but eacht time I try the sorcerer it fails.
After zoning again I can mostly (not always) apply guard again.
I got the same issue on Assassin, sometimes on zone in, but always after death. It claims target is guarded but it acts like its not. The only way is changing zones again.
Interestingly enough in such situations I can guard any other group member just not the one I was guarding. Did not think its related to sorcs, but yeah sorcs in my case too, if it makes a difference.