How to scale multiple objects on a given axis? I made a simple model of a house (fig.1) .
Now, I want to copy the house and mirror it negative 1 on the Y-axis in global orientation.
It should give me an exact mirrored copy of the original but instead all of the different objects get flipped in unwanted directions (fig.2) .
The house contains multiple objects (roofing, windows, doors, etc.)

I've run into this problem once or twice in the past but did not find a solution or why this is happening, nor could I replicate it in a clean blendfile.

(I know about the transformation orientation settings as well as the different options to set the pivoting point of one or multiple objects.)

Does someone know why this is happening?
Parent the duplicates to an Empty and then scale that empty by -1 on one axis.