Layout has no special support for isometric drawings. It measures the scale on lines parallel to the image plane, as you want in a typical orthogonal drawing. In an isometric drawing the scale should apply to lines going diagonally away from the camera.
To get isometric drawings to work in LO you need to multiply the scale factor by “how much” these lines diverges from the image plane.
From some fiddling with a cube and diagonals representing the camera direction you can find the factor
0.8164965809277244. I use the Ruby API to measure precisely, but it is something you can write down and reuse. You can probably also do the trigonometrics and find an expression for this factor but I’ve been too lazy so far.
Once you have this factor you need to set up custom scales with it in LO. Enter a scale like normal, but multiply the denominator by
0.8164965809277244. For instance 1:100 would be 1:81,64965809277244. Remember to change the scale name, in this case to 1:100 (Isometric) or something similar. 1:81,64965809277244 isn’t very descriptive.
Also be sure the view is actually isometric. LayOut’s built in isometric view is NOT isometric. Neither is SketchUp’s if you set it while being in perspective projection. Make sure a cube is shown as a perfect hexagon, otherwise the scale will be off.