The scale of the component does not reach negative values like the instance of the object. However, the user is able to enter a negative number and the object correctly reflects that entered number. To make it easier to see the negative values, place a material like M_Chair from starter contents. Negative values would flip the texture on the axis.
You are able to scale it back up from 0, it just takes a little while since it is scaling from a very small decimal number.
This issue also occurs in Blueprints Component Editor
If the user is not allowed to go into a negative value, I would expect there me a message, or a warning stating that it cannot happen. Or when entering a negative number, it is converted to a positive number.
This also occurs in Main, 4.14 - Not a Regression
Result: The value stops at 0
Expected: The value of the cube goes into the negative value
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-38968 in the post.