Weird animation first frame importing from blender

Hello,

I made my first model in blender and I animated it. It looks fine when I play the animations through blender, but when I import it in unity the arms are broken on the first frame:



So I messed around a bit in blender and it seems like the problem is from there, because I can reproduce it. If I go from the t-pose straight from to the run animation, I get this:



it's pretty much exactly the same thing. It's like the top of the arm IK doesn't follow the shoulder and stays where it was in the t-pose. Note that it ONLY happens when going from t-pose to run animation, If I then click on keyframe 0 I get this:



I also don't ever see this bugged frame when I loop through the animation with blender. If I make the unity animation start from frame 1 the bug doesn't appear, but then the transitions aren't perfect.
This only happens with the arms.
Here's my arm's setup:
IK on forearm + upper arm. no deformation. upper arm's parent set to the shoulder. Target set to hand. and there's a pole behind the character.
2 smaller bones for deformations. set to follow location + rotation of ik bones. upper arm deformation's parent set to the shoulder.
shoulders follow rotation of upper arm, with limits to rotation.

Does anyone have any idea why this happens?

 

a temporary solution I found was to, in Blender, duplicate the first frame and put it on frame 0 and then make the animation skip the first frame in Unity.

If anyone has a real (non-hacky) solution please let me know, because I'm pretty sure this problem stems from a lack of understanding of Blender.

Thank you.