The report that speaks of twenty rak‘ahs is extremely weak and it is impermissible to rely on it.

Then, in Fatḥ al-Bārī (4/205–206) under his commentary on the first hadith, Ibn Ḥajar writes:

10 – (very weak)

“As for what Ibn Shaybah transmitted from Ibn ʿAbbās—‘The Messenger of Allah ﷺ used to pray twenty rak‘ahs in Ramadan and then the witr’—its chain is weak, and it is contradicted by this report of ʿĀʾishah that is in the two Ṣaḥīḥs, for she knew the Prophet’s ﷺ night practice better than anyone else.”

Al-Ḥāfiẓ al-Zaylaʿī had already made the same observation in Naṣb al-Rāyah.

I say: This hadith of Ibn ʿAbbās is extremely weak, as al-Suyūṭī stated in al-Ḥāwī li-l-Fatāwā 2/73. Its defect is the presence of Abū Shaybah Ibrāhīm ibn ʿUthmān, whom al-Ḥāfiẓ (Ibn Ḥajar) judged in al-Taqrīb to be “abandoned in hadith.” ⦗22⦘