When the text in spinbox has focus, it doesn't get the change notification externally. It may also be worth noting that ctrl z or ctrl y also do not get bubbled up to undo the text box values.
It appears that the undo transaction does not revert the transform value back to its original value if its text field still has focus when the undo takes place.
In order for this to work as intended, you would have shift focus away from the text field (like select the Rotation Y Axis text box after Step 3) before you go to the Edit menu to undo the rotation.
We found an issue with Undo when transforming multiple selected objects. Here are the repro steps:
1) Add two static meshes to the world.
2) Select both meshes.
3) Go to the Transform Details Panel and enter 90 in the Rotation Z Axis text box and press Enter. This will rotate both meshes 90 degrees along their Z axis.
4) While the focus is still on the Z axis text box, go to the Edit menu and select Undo Set Rotation.
5) Note that while the two shapes will revert to their original rotation in the viewport, the Rotation Z Axis text box will be highlighted and still show 90 instead of 0.
6) When you shift focus away from the Z axis text box (ex. click on the Y axis text box), the static meshes will then rotate 90 degrees along their Z axis again.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-134971 in the post.