Software Details:
Windows 10 / Ver. 1809 / OS Build 17763.316 / Illustrator Ver. 23.0.2

Steps Taken:
a. Selecting Warp tool (or any other from the warp tools palette)
b. Selecting an object
c. Warping the object either once or repeatedly
d. Undoing all warps to restore original shape.

Expected Result
The undo command would undo my actions

Actual Result
Although the outline that appears when hovering over the object had returned to the anticipated state after the necessary number of undos, the object itself remained unaltered - sometimes disappearing. Subsequent undos would only affect prior actions on other objects. Only after expanding the warped object's appearance and then undoing that operation was I able to achieve the desired result.