Resolved by author
Overview of the issue
When using the JetEngine plugin and its Relations feature on a site with two languages, there is an issue with the relationship between custom post types. Let’s say that we have two custom post types, for example, shop and coupon. If a relation is set between those two custom post types in default language, and then the shop post is translated, the relationship is unexpectedly moved to the translated shop post.
Translate your custom post type (coupon) before setting the relationship.