Investigate why unique_relation? method returns true unexpectedly

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See discussion !107498 (comment 1244232521) and method https://gitlab.com/gitlab-org/gitlab/-/blob/0dd1428d48345d9d2252460c14e7a60c38b5fc7d/lib/gitlab/import_export/base/relation_factory.rb#L298

With some models unique_relation? returns true unexpectedly. The goal of this issue is to figure out why and fix it if possible.

follow up to: #390743 (closed)