Can you change the video to unlisted so that we can view it? It's set to private so we can't see it.
What I do anytime I'm switching between two clips is set a keyframe for the character position before the change in clips. I don't see you having any keyframes before the next "walking up stairs" clip comes into play.
Try key framing the starting position (before your character moves up the stairs) and on the first frame of the "walking up stairs" clip, position the character to where they were before that clip (which creates another keyframe at the start of the "walking up stairs" clip). Then you can set the ending keyframe (end of walking up stairs clip) and reposition the character to where you want them to be as they get to the top of the stairs. When you play it back the character should follow a direct route up towards the ending keyframe.
If you actually want to show your character's knees bending as he actually walks up the stairs, then you need to take the plane prop A, and create stairs with it. Size the planes just like your stairs in your set and position them over the stairs. Then convert them to terrain. That way when your character walks up the stairs, his foot contact actually connect with ground.
IMDB: James Muia
Aviticus Dragon on YouTube
Intel® Core™ i7-7820X Processor (8x 4.30GHz/11MB L3 Cache)
Corsair Hydro Series H115i 280mm
Liquid CPU CoolerASUS ROG STRIX X299-E Gaming Motherboard
32 GB DDR4-3200 GSKILL Ripjaws V Memory
NVIDIA GeForce GTX 1080 Ti - 11GB - EVGA FTW3