I'm no expert, but I'll tell you what I see in all videos..The first one, the dog Kira is playing with looks like a big doofy boy/girl,,to me he's showing just that.
the 2nd video, Bruno is down in his "stalk" position,,now he may just wanna play, but I would say, he looks intimidating to Kira..heck I'd run to, if I had Bruno in his 'stalk/stare' zero ing in on me..
Third,,while he's playing, Bruno that is, he looks like a 'rough' player,,tail up, just going at things full boar..
I'm thinking Kira may be a tad intimidated by his size, his stature, just the rough vibes he may be giving off..You notice in the 3rd vid, I see her just come into camera range with the smaller dogs, and Bruno comes charging thru, she gets out of his way.
The first dog she was racing around with, you could tell she was very comfortable with him, the next two, unsure/uncomfortable with bruno..Kinda can't say I blame her, not sure she 'knows' how to take him