These gliders are strange creatures, lol.
There are many, many reasons gliders will bite. From what you described, Sydney sounds like a nipper. Nipping usually occurs more so, in my experience, after they are more bonded or comfortable with you and either don't like something you are doing, or you have due to a scent picked up on you. But sometimes there are the nippy gliders. I have a boy who is like that. It may calm down over time still, I think my boy is getting a bit better with it. I'm not sure if it's a testing nip, but from how you described Sydney, it sounds like it's basically she's a nipper. Try tsk tsk or hiss at her when she does that, like they use for correcting, that may help.