I have done some more testing here, and I can confirm that I see this problem behaviour with Relationship field and the Event Calendar tribe_events post type when the Polylang multilingual plugin is Enabled. I tried updating Polylang to latest version, and the problem still persists, so Polylang must be causing problems with the REST API query for post lookups in the Editor, I guess.
I’ll see if I can see any way of overcoming this in Polylang settings.
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