Snap to grid shortcut does not work if the decimal value of the (X or Y or Z) coordinate is set too low.
Adjusting the Y or Z value after setting the X value to -64.000038 will allow the object's X value to correctly snap to -64.0
Result: The object does not snap to an even -64.0
Expected: The object to snap to an even -64.0
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-90931 in the post.