When you mirror in Capture it is not the model of individual objects that gets mirrored, but rather the placement or arrangement of all the objects you have selected. The main reason for this is that it is impossible to mirror fixtures as well as many truss items. The kind of mirroring you were expecting would be the one found in a 3D modelling tool where you are defining the shape of an object rather than placing objects.

In my tests (V 2025. 1. 10) nothing will mirror in the true sense of the word except a fixture attached to a hanging structure.
For example a truss rotated at 45° in top view doesn’t mirror, but if a fixture is hung on it, both the truss and the fixture mirror properly.
Or for example a bunch of booms SR with focused fixtures hung will mirror properly over the centre line to SL, but the wheelbarrow ( from objects / outdoor) merely translates its position. This is absolutely not what I’d expect; It’s been driving me crazy..
At the very least this behaviour should be noted in the documentation. Is this the way it’s going to stay?
Honestly! - haven’t you guys read ‘Alice through the Looking Glass’?

Cheers
rabbit

For the time being this is what it is, yes. I will add that if we do add model mirroring one day it is not going to allow you to mirror a piece of truss because it's not possible to mirror objects in real life, model mirroring would only be available for forms.