I cheked this with a new LO document and yes I can refresh the bibliography. Not only that, but after I created the bibliography list for the first time, it automaticaly refreshes the list along with every new citation I add! (I don't know if that's supposed to happen, I don't remember this happening before).
So I guess the reason "refresh" doesn't work in the current document is because something happened when I saved it as .odt from .docx. The only thing I can notice in the "corrupt" document is that the style is marked as "Bibliography 1 (user)" while on the new one is as "Bibliography 1".

@Nikos_EAP You should see the aborel's link above, and make sure you transferred your document with Zotero citations from Word to LibreOffice correctly.

Regarding bibliography updating automatically, this is controlled in Zotero Document Preferences by the "Automatically update citations" checkbox. If you were working with Word before, Zotero may have prompted you to disable automatic updates to improve citing speed. Zotero offers to disable automatic updates when working with a sufficiently large document and citation updates start becoming slow.