I apologize for the delay in getting back on this, as we had an unusually busy forum queue over the weekend.
During troubleshooting, I noticed that the view "Projekty studenckie > opiekunowie projektu" was set to show all the posts/items from the "Opiekunowie projektu" repeatable field group.
As we only need to show the "Opiekunowie projektu" repeatable field group items, which are related to the current post, I included a relationship query filter in the view to only show the related posts, which removed the extra/non-related items.
( screenshot: hidden link )