Titan Boss bug

In some instances with the massive bosses, like Titan.

The boss will stop attacking, stand still and let you kill them.

I think it has to do with a reset in its tracking between attack sequences,

If I stop moving, after one sequence is finished, it needs me to move in order to track my position again?

not sure.

I was also standing as close as I could to it but stopped moving while attacking it.

I believe this can happen to another massive boss, The giant skeleton thing near the end of Act 2.