I've emailed you a test scene as requested as it is too big to attach here,
You will see from the camera view I have mirrored the hedge geometry from the left to the right.
This is the exact process I took:
Hide forest pack objects, select and group hedge geometry, adjust group pivot to mirror, mirror an instance, ungroup all, add respective new geometry to respective forest pack.
You will notice the forest pack Forest HEDGE TIDY SIDES is inverted on the left hand side.
(Side note: I have noticed the forest pack on the taller curved hedge section was already inverted on the left hand side before I mirrored it, but the rule still applies it is inverted on the right hand side)
The remaining forest packs are using surface area mode and these aren't working at all on the mirrored geometry.
Thanks for your help