View with multiple value fields: suppress display of repeated column values
The answer was obvious. For the Entity Reference type of field, don't use fields from the referenced entity (like e.g. "Title") from a relationship. Instead, use the field from the main entity (without relationship). As a result, it displays a title of the referenced entity anyway and not an ID or something.

But in this case, the field gets "Multiple Field Settings" config group, which should be set to "Display all values in the same row". I saw that solution before, but in my view there wasn't this config group at all. That's because I used a field from relationship, which apparently doesn't have a setting for multiple value, because in that entity it's single value, it's a title.