So, it seems that this issue is a known bug, which has not been fixed yet.
I have been told that, as a workaround you can download and use James Murray's Shape Tag from BIM Components.com, which works fine with mirroring.
https://bimcomponents.com/LCF/Details/832
I haven't tried this yet but thought I would share if anyone is having the same issue.
Kathryn
Archicad version 19
Windows 10