Mirroring algorithm is not that simple with LEGO elements to provide a good outcome and additional metadata shall be attached to each element one by one. However it is feature that will be worked on in the next year or so.

Awesome, thanks.

In blender i could mirror the blocks easily enough without a problem.

No it is not the same principle. In Blender you can use a negative scale to mirror an object. But that is not a real life thing. The proper algorithm would be to calculate the rotation and position and replace some elements by their opposite when required. E.g. 24307 and 24299. It also depends on the mirroring plane as well as other parameters.

i never said it was proper real mirroring.
visually it works for 3D renders in blender but yeah doing the same in mecabricks is out of the question.