It sounds to me like you may be encountering Prismic's surprising behaviour when a UID is changed.
If a document had a UID, say "old-uid", and you change its UID to be "new-uid", the document keeps the old UID in its history. This means that the old UID value cannot be used by another document, even though it isn't currently in use by any document, and it also means that the document that used to have that UID can still be fetched using the old value.
We hit this issue a few weeks ago and the only solution I found to free up the historical UID so it could be used by a new document was to find and delete the original document, after first duplicating it to keep its content. Deleting a document seems to be the only way to make it release the old UID value(s) it has had.
Your best bet to find the problem document is to fetch it using the UID (SKU) then check the uid field in the returned document: it will be the current UID of the document not the value you used to fetch it.
See also this discussion: I changed the UID, but page with old UID is still active?