I’m not very good when it comes to the details of the grades of traditions. Is it a forgery because one of its narrators is rejected? What you think are the reasons an author would include it in his book when the status of a hadith is highly doubtful?
There are many reasons some internal some external. Among the internal reasons are poor Arabic, run-on narrative, contradiction of established texts, maybe clashing with the known style of the Prophetic hadiths and other aspects that trained eyes may pick up. Among the external reasons are the fact that there is no chain of transmission; al-Riqashi was not a trusted student of Sayyidina Anas and no one relates this other than him; also al-Samarqandi is more of a jurist and admonisher but was criticized when it came to hadith, as he did not discern between them.
His books were redflagged by early and late authors as unreliable for hadiths, so something that is only found in them is unconfirmable. All these are telltale signs of forgery. Not so long ago there was discussion of a narration found only in al-Daylami. This is by itself a telltale sign because a narration cannot just materialize out of the blue in some isolated weak and/or late source while everyone else under the sun has never heard of it, and then be considered authentic. This is an even weaker source and then there are all these other signs as well.
In his book al-Majruhin Ibn Hibban listed twenty types of unreliable narrators. Among them:
“The fifth type were overall pious worshippers who were unconcerned with memorization and discernment. Whenever they narrated something they would attribute to the Prophet œ what he did not say, or connect disconnected chains of transmission, reversing their order, and mistaking the sermons of al-Hasan, for example, for hadiths narrated from Anas, from the Prophet (s). Their narrations were no longer considered proofs. …
“The eighth type are those who lie unintentionally and unwittingly, simply because he is not a person of learning and he has no idea what learning is.”
These two types are one of the reasons why Imam Malik said his famous phrase “No one lies more than the pious.” See more in my Four Imams.
Hajj Gibril Haddad