Verified after latest hotfix that 5/5 upgraded legendary offhand no longer selectable for re-imprinting.
Additionally, transmog does not stay applied to offhand and immediately reverted in open world when moving items in inventory.
Continues to be an issue since launch.
dealing with the same issue. From what I seen it’s always a book off hand, imprinted once and it won’t let me imprint again. i transmog it, but it switches back.
Tested it on a random garbage sacred, couldn’t re-imprint on a non-upgraded sacred legendary either.
The skin not staying applied:
Bug with the item - it does not exist → the game removes the skin → there is an item → it will be shown (with its normal skin).
It is possible that you do NOT get the bonuses from said items. (Your attacks are weaker, …)
Re-imprinting:
Let me correct the text below Maybe not. It works now. 
It might be wanted behavior that you only can imprint an item once and that you cannot extract an aspect used for imprint.
(There are some threads based on players getting the wrong(?) message. I got different messages when I threw in random imprinted items to check the messages before the removal.)
I can change imprints on other off hands just not ones that are books.
Same been having this exact issue for about 4 days or so.
Same problem. It’s the book type as well.
same problem with me its a book type… blizz please fix it
Still not fixed or acknowledged despite patch 1.04 release today.
SAME HERE… book type