How to reproduce:
1. Open the attached project "colliderBug.zip"
2. Open the "SampleScene" Scene
3. Expand the "Group" GameObject in the Hierarchy window
4. Expand both "Sphere1" and "Sphere" GameObjects in the Hierarchy window
5. Select the "SpikeNoRig" GameObject in the Hierarchy window
6. Observe the selected GameObject's Transform Component in the Inspector window
7. Enter Play Mode
8. After 5-6 seconds repeat the 6th step

Expected result: Transform Component's rotation remains (-90, 0, 0)
Actual result: Transform Component's rotation keeps changing on the X-axis, while rotation on the Y-axis is set to 90 and to -90 on the Z-axis (Might be -90 on the Y-axis and 90 on the Z-axis on 2020.1, 2020.2)

Reproducible with: 2018.4.27f1, 2019.4.9f1, 2020.1.4f1, 2020.2.0b2

Notes:
- The issue is also reproducible when the GameObject's rotation is getting set to the same Quaternion.Euler value on FixedUpdate
- "SpikeRig" GameObject is identical to the "SpikeNoRig" GameObject, it just has a Rigidbody Component with "Is Kinematic" property set to True, which gets rid of the issue
- The changing rotation of the "SpikeNoRig" GameObject also has an effect on the parent "Sphere1" GameObject's Transform Component
- The issue is not reproducible if the rotation is set to a Vector3 value instead of a Quaternion value