I am currently modeling in Blender 2.79b. I wanted to use Version 2.8 to render, because it supports CPU and GPU rendering at the same time and is faster. I experienced a difference in a model in pose mode. I used the model https://www.blendswap.com/blends/view/87031 (Zootopia Fan Art Nick Wilde) in a modified version. I moved his hand to let him touch a carrot. In Version 2.9b he touches the carrot. In Version 2.8 he does not lift his hand up high enough.
Steps to reproduce: Open the blend file. Change current frame from Frame 0 to Frame 1. Observe the hand position of the fox. If the file is saved directly on frame 1 it works in both blender versions in the same way. But if you go away from frame 1 in blender 2.8 and back to it, you get the same problem with the hand position as when starting on frame 0.