To reproduce the issue mentioned:

Download and extract the "IK_Bug_Example.zip" file attached by the user.
Open the Unity editor.
In the Unity editor, navigate to "File" -> "Open Project" and select the extracted project folder.
Once the project is loaded, locate the "SampleScene" under the "Assets/Scenes" folder in the Project window.
Double-click on "SampleScene" to open it in the Scene view.
In the Unity editor toolbar, click on the "Play" button to enter Play Mode.
Expected result: The "Bone_Thigh" GameObject should not rotate.
Actual result: The "Bone_Thigh" GameObject rotates.

Note:

This issue has been reproduced in Unity versions 7.0.8 (2021.3.14f1), 7.0.9 (2021.3.20f1), 9.0.0 (2022.2.0b14), 9.0.1 (2022.2.9f1), 10.0.0-pre.1 (2023.1.0a24), and 10.0.0-pre.2 (2023.1.0b6, 2023.2.0a5).
The issue could not be tested in Unity version 2020.3.45f1 due to an error importing the prefab.
The issue has been reproduced on macOS 13.0.1 (Intel).
Additionally, it's worth noting that the "Bone_Thigh" GameObject can also rotate when selected in Edit Mode and when the mouse pointer is moved between different Unity Engine windows. The rotation can also occur by simply selecting the "Bone_Thigh" GameObject in the Hierarchy.