i got a Weird Problem since i updated to 2.91, then i try to loop my animation by changing the interpolation type to Linear the Result almost looks the same as Bezier, i did try it with a new scene and a simple Cube with a Armature and a single bone. the Animation seem to be faster with Linear but the end and start from every key frame is still eased in/out.
The problem is that you're using Quaternion (WXYZ) Rotation, which is the default setting for bones in Pose Mode. Set the Rotation method to XYZ Euler and animate the Y Rotation from 0° to 360°.
You should also set the last keyframe with 360° on frame 101 instead of 100 if you want a looped animation from frames 1-100, because 0° = 360° and the animation would look as if it pauses for one frame.
So the reason why it never occurred to me before was because i was using less keyframes for my Animation, it seem to be the case that Quaternions starting to slow down and wooble a lot then your animation is longer than 100 Keyframes. So for the Future i will use Euler in Technical Animations for persistent Animation Cycles.