Blender seems to reset keyframes I am new to animating in blender and have a (what I think is) weird behaviour, which makes animating very frustrating and near impossible for me.

Please watch the uploaded .gif: https://imgur.com/nkLGrZ7

First I show my animation. After that i select one IK element in Pose Mode and play the animation again. Please notice that some positions of my bones just change in the animation. I did not modify anything. Since I bought the AK  model online I don't think I am allowed to share the blend file.

For easier comparision here are two pictures. One before clicking on the IK element and one after (same frame in the animation).





This also happens everytime I move something in pose mode. Some positions change on other frames and I can't controll my animation.

Is this a known issue or am I missing something?

Thanks in advance.

EDIT:
uploaded the file without the ak. The arm rig and model is from this video (https://www.youtube.com/watch?v=xzLnkpBQUfA), so I hope it is Ok to share this here. 



Steps on how to easily recreate an issue: https://imgur.com/a/2qK00D2
Okay, so I downloaded blender 2.8 a few days ago and just tried to open my animation with this version and it seems to be fine. I don't know what went wrong with my 2.79 version, maybe it is some problem with my local installation...

EDIT:

Nevermind, the problem still persists, however, now I am able to get rid of it by pressing STRG+Z sometimes.