-=[Disconnected Reports]
Yaḥyā bin Saʿīd said: Al-Zuhrī’s disconnected reports are worse than those of others since he is a ḥāfiẓ. He would name whoever he could name, but neglects to name those that are not proficient or those that he couldn't bring himself to mention.

Tārīkh Dimashq 30/227
So many things happening here...
First of all, be aware that Yaḥyā is a major critic of narrators. Al-Zuhrī is a second generation Muslim and an Imam in hadith. This didn't prevent Yaḥyā from weakening his disconnected reports.

No, just because he is from the second generation doesn't mean that he necessarily got that specific report from a companion. Sometimes, there are multiple tābiʿīn narrating from one another.
First hadith in Bukharī has three of them.

Al-Zuhrī's proficiency is a reason for his disconnection to be more problematic than others. Other narrators may have simply forgotten the name of whoever told them the report. Al-Zuhrī usually didn't, so Yaḥyā treated those disconnections as intentional.

One of the reasons for intentional disconnections is because the narrator is aware that the source is unreliable.
Choosing not to mention the source doesn't affect the student's truthfulness since not mentioning a source isn't lying.