Hi,
A Mirrored feature becomes completely independent after the mirror. There are
two ways to modify it. You can either rewind the Timeline, make the change and
then wind the timeline back and the change will be reflected on both sides of the
mirror; or you can direct edit the mirrored feature, but this will NOT reflect on
the original.
There are also times when you will deliberately want the two sides to be different.
Take a Boat model. The basic hull and deck will probably be identical on both "halves"
but there will be certain fittings that will be different. The anchor box will be smaller
and different to the Sail Hatch. One is a literal box with a hatch and the other is a
hole with a hatch. They need to be different.
If you need to make modifications that are reflected on both sides of a mirror then
you should be selecting the original feature to edit anyway and so the timeline does
actually show that selection. One of the reasons that a mirror is NOT expanded in
the timeline is to stop the timeline becoming huge by doubling it with a single command.
The reason that a feature on the copy is not highlighted in the original timeline is
because it can be changed and edited and so is NOT the same thing.
Cheers
Andrew