Does she have the same issues with eye contact when sitting down? Disparate heights between adults and children can make children uncomfortable and feel inferior when trying to maintain eye contact.
I'll project some of my personal experience, in case my lens rings true for her:
A highly aware visual person can get overwhelmed by the sheer quantity of often contradictory information that comes from another person. Some adults have poor performance control when dealing with kids such that their non-verbals and verbal content are completely out of whack such that looking at them while they speak is a bit disgusting (closest word that comes to mind.)
The delays and such particularly with strange adults may be a very introspective overthinking thing where she is aware of tone of conversation, content, what impression she is creating, norms for behavior as expected by adults. She may need the additional focus to form an appropriate response that conveys the right level of conversation for the audience. If you are also seeing this, there may be a performance component so she is seeking a tone and content that maintains consistency with the social face she presents to you as well as finding a negotiated level.
Other delays can come from trying to be polite and finding the right transition moment and make sure someone isn't just taking a long pause.
Her default level around other kids may be her best practiced social face. The unfettered blurting thoughts and such may mean she is working as close to natural as can be.
Other factors I find with watching someone's face when they are speaking is sometimes I get really distracting feedback delay loops (ever tried listening to your voice on a half second delay through earphones while speaking?) And sometimes it is a distracting detail in their face.
But if a speaker wants my undivided attention, then it's best I not look while they speak.
My personal work-around is to make sure I use very active listening skills like nodding or verbal tiks and asking strong, insightful summative questions.
