Ideally, for a ‘bug’ to be resolved it requires replication.
The. Purpose of the following is to present an opportunity to replicate the ‘bug’ or show that altering the line dimension adjusts in the expected direction. If you follow the directions the result may determine a route to a satisfactory conclusion or if errors are reliably repeatable present them for investigation to the S3D Team.?
Note that all Snaps are Switched On.
In the ScreenShot below the Triangular shaped Closed Sketch [on the XY Axis] was used to Add > Construction Plane > Offset, in this case 50mm [Plane 01]. This Closed Shape could equally have been a Triangular Body.
The Triangle Base was Tools > Projected onto the newly created Construction Plane.
This new Construction Plane was activated and squared up by Tapping on it with a Finger.
Note that the new Construction Plane must be used to form the Rectangle.
At this stage the Projected Line of the Triangle Base was horizontal.
Then from the Righthand End of the Projected Line a Vertical Line was drawn Downward, in this case on Grid, for less than the required 100mm.
The Line was adjusted using the pop up Dimension Input Tool and as expected increased to the required 100mm, i.e., the Start Point of the line remained static and the Line extended from the Start Point.
Each of the Remaining Rectangle Sides were similarly properly formed using the last drawn End Point as the new Start Point.
Opportunity was taken to obtain similar results with initial dimensions of 15mm and 75mm.
At no time was it necessary to Lock any Point(s).
I have run this through a few differing scenarios and have not experienced anything unexpected:
A long time prior to these recent examples I experienced similar situations to those of both @tommyn and yourself. However solutions are not realistic without replication, an aspect that requires considerable effort.