Not_found error on update

I’m getting errors on updating a document and I can’t work out why. On app startup documents are replicated from a server, then loaded. A document can be selected by the user and edited. However at some point updates stopped working. I have double checked _rev values in the latest saved document and update are matching. I am getting
{ status: 404, reason: 'not_found', error: 'not_found' }
on update after editing a document.

Are there any pointers that might help me to debug this?

I am using the ReactNativeCouchbaseLite module, which employs CBLListener.
Just updated to CBL 1.2.1-13 (problem exists both here and in 1.1.1)

My bad, I was simply giving the document to the documentUpdate function - it expects docId and docRevision separately.