I experience a problem similar to what is described here: https://wordpress.org/support/topic/plugin-polylang-other-language-post-ids-do-not-persist-between-saves?replies=6.
I have two languages. When I add pages to my site using the "plus" to create a new page in the second language to the existing page in the first languae, the PageID, which links the two pages does not seem to save. As soon as I hit "update" it is gone.
language set from content
all other plugins deactivated
static front page
Hope anybody can help