The Default Values in the Blueprint are set up correctly. Adding some more info, the issue here is not the default Value in the BP itself, but rather clicking on the yellow default button always sets the struct value to '0' , regardless of the Default Value assigned to the struct in the Blueprint.
When using the track builder plugin, the default settings for the track width starts at 1. However, when the yellow arrow is clicked to to restore the track back to default it sets it to zero instead.
The value resets to 0.0 as opposed to 5.0
Note: If you Open the TrackGenerator BP and the Road Data Struct you'll notice that the Default TrackWidth is set to '1' and not 0.
Head over to the existing Questions & Answers thread and let us know what's up.