5 Week Abandoned Kitten Having Fearful Moments

A week ago, a neighbor found an abandoned kitten and gave her to my partner and I. Kitten has seen two vets who both think she's healthy and aged her at four to five weeks when she was found, so she's now five to six weeks old now. She's eating well, getting more and more playful with toys and with us, figuring out how to play with the resident cat, and getting to be very curious. In the last couple days, she's had a couple moments where she gets a little frisky/playful or is exploring, and then seems to become very fearful - runs away from something with her tail puffed up, back arched, hissing. I haven't been able to discern a trigger. She comes to me and lets me pick her up right away after, and the fur will go down after a little petting and soothing words. Does not seem to be fearful of my partner or I at all, ever. She's ridiculously prosocial with people in general. If anything, the fearful moments seem to be when she's exploring a new part of the apartment during playtime.

Is this a normal part of development at this age? Any recommendations? I'm more used to dogs than cats, but I know puppies have "fear stages" where they're extra sensitive to things. Wondering if that's what's going on with the kitten? Just want to make sure she's set up to have a healthy and happy life and want to intervene asap if this is concerning.