The modifier works with the original data of the mesh as long as the scale is not applied.
So, let's say you have an object of 1 m. Scaled by 44 as your object, this is now 44 m. If the displacement would be set to 0.1 m, this results in 4.4 m displacement. Of course you can't set "0.1 m" in the modifier, just a strength, I just mean so that it would be displaced 0.1 m if the object was 1 m in size.
If you now apply the scale, the 44 m will become the original size of the object. But the Displace modifier doesn't automatically change its settings. So the displacement will be 0.1 m like before. Now this is very less displaced than 4.4 m compared to the overall size and in the case of your object with just a few milimeters it might even be hardly visible.
That's why it's always important to apply the scale when working with modifiers or tools that are using the original base mesh data.