Indeed, there are quite a number of weak hadîth in al-Adab al-Mufrad.
We must understand that al-Bukhârî wrote a number of books. Only one of these – his most ambitious work – he gave the title al-Sahîh, meaning “Authentic”. He imposed upon himself the most rigorous conditions of authenticity in writing this book. He would not record in al-Sahîh any hadîth unless it fulfilled those conditions.
To make sure he succeeded in his goal, he aimed far higher in his Sahîh than the minimum accepted conditions of authenticity. This ensured al-Bukhârî that if he made a “mistake” – which is inevitable in any human endeavor – that would mean his recording in his Sahîh a hadîth does not fully meet those extra-rigorous standards – however, it would still be an authentic hadîth nonetheless.
He did not set these strict conditions for the other books he wrote or compiled. He did not even set the condition of the hadîth having to be authentic. He would allow in his other books hadîth that were only good (hasan). He would allow borderline cases. In some instances, he would allow weak hadîth.
This is the case with al-Adab al-Mufrad. It is a book about “adab” – about good manners and virtuous behavior. It is not a book about Islamic beliefs or matters of law.
This is why al-Bukhârî is willing to mention even weak hadîth in al-Adab al-Mufrad. Weak hadîth can be tolerated, because the book is talking about exhortations and virtues. The scholars used to tolerate the narration of moderately weak hadîth regarding virtuous behavior, but they did not tolerate weak hadîth for establishing legal rulings.
Please also know that of the hadîth in al-Adab al-Mufrad that you might hear some critics describing as weak, not all of them are the same. Some of those are borderline cases. Some might be considered good or even authentic by al-Bukhârî or other hadîth scholars. In a book which is not written upon the most rigorous standards of authenticity, there is bound to be a number of hadîth about which specialists will disagree regarding their quality.
Allah knows best.