Possibly all versions?
Short description of error
While working on a project, I discovered an inconsistent behavior in the transformation constraint.
Because drivers have no way of referencing "self" value (as done in After Effects expressions, which is a needed feature, in my opinion) I found myself using the transformation constraint a lot to be able to manually alter a transformation parameter (location/rotation) beyond a driven value.
But that's the thing: I can have control of location and rotation, but not scale. As soon as the scale parameter is set, it's locked unlike location and rotation.
I'm not entirely sure if this would be more akin to a design flaw than a bug, but I thought I'd report just in case (couldn't find a discussion of this anywhere).
Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps