A key caution concerns the first-heard hadith series: the text states that the firstness feature is traced only to Sufyān ibn ʿUyaynah. Extending that distinctive feature to the very end of the chain without evidence can be an error.
This separates continuity of the isnād as transmission from continuity of its repeated feature. A chain may remain intact even though the feature of being each narrator's first hadith is not established throughout.
If firstness is proven from Zayd through Bakr but not for an earlier link, it is accurate to describe that proven portion rather than claim the entire chain is musalsal bi-l-awwaliyya.