The reason for al-Bukhari’s reservation is that he himself recorded the report in his History (al-Tārīkh) through the chain: Ismāʿīl ibn Abī Uways → his father → Mūsā ibn Ibrāhīm → his father → Salama, thus adding an extra transmitter to the isnād. Al-Ḥāfiẓ comments:

“It may be a case of an additional link that has slipped into an otherwise continuous isnād, or the explicit ‘hearing’ stated in ʿAṭṭāf’s version may in fact be erroneous; this is the aspect that invites scrutiny of its chain. Those who declared the narration sound relied on the version of al-Darāwardī and considered ʿAṭṭāf’s route supporting evidence for its continuity.”

I add: Mūsā ibn Ibrāhīm explicitly affirmed that he heard it from Salama in the version transmitted by al-Darāwardī and recorded by al-Ḥākim. Al-Ṭaḥāwī’s wording, however, comes from al-Darāwardī through Mūsā ibn Muḥammad ibn Ibrāhīm, from Abū Salama.

This Mūsā ibn Muḥammad is Mūsā ibn Muḥammad ibn Ibrāhīm al-Taymī, Abū Muḥammad al-Madanī—a different person from Mūsā ibn Ibrāhīm, and the latter is weak.

Accordingly, al-Ṭaḥāwī’s route is anomalous; he reported it through Ibn Abī Qubayla from al-Darāwardī, and I do not presently know who this Ibn Abī Qubayla is.

The report also has a corroborating mursal and disconnected version. After citing the ḥadīth, al-Bayhaqī says:

“ʿAbd Allāh ibn al-Mubārak related from Ibn Jurayj, who said: I was informed by Yaḥyā ibn Abī Kathīr that the Prophet ﷺ forbade a man to pray in a shirt whose buttons are left open, out of fear that…"