Same issue here, I’ve created (and placed inside my scene) a simple actor blueprint with a custom variable of actor reference type and I’m unable to assign anything to it in my scene.
I also checked this answer but doesn’t solve this issue: Unable to assign Actor to editable member variable in editor details panel (UMG) - Programming & Scripting - Unreal Engine Forums
Edit: I found the issue, my actor was in a different streaming level than the rest of the scene and this was causing the issue.