Grease Pencil - Mirrored Version not avaible

hey guys.
I ve just applied the mirror modfier as you can see in the screenshot. Now the mirrored version isnt avaible to edit. I just can select the already existing version and if i move it the mirrored version disappears. Duplicate is not possible too. After that i just get the error Message “not implemented”.
Maybe u guys can tell me what happened there and how to fix it.
Sincerly Alex :slightly_smiling_face: