Is food that is originally permissible, but of unknown source or method of slaughter, considered among the doubtful matters that are forbidden?

Scholars differed concerning the meaning of "doubtful matters" mentioned in the Hadith (definition), and it has been interpreted in four ways: conflicting evidences, scholarly disagreement, disliked (makrooh), and permissible (mubaah) that is not entirely balanced between its two sides. The preponderant view is that it refers to conflicting evidences, which is close to what Al-Nawawi explained it to be: matters whose permissibility or impermissibility is not clear.

As for Chinese food: Whatever is made of plant-based ingredients and is not mixed with alcohol or impure meats is clearly permissible (halal (definition)). Whatever is mixed with meat of permissible animals slaughtered according to Islamic law by a Muslim or a person of the Book, and was not offered to other than Allah, is permissible. Whatever is mixed with pork or alcohol, or its slaughter was not according to Islamic law (from a Magian or a Buddhist, or by stunning), is clearly forbidden (haram (definition)). Whatever is mixed with meats whose slaughter is not overwhelmingly presumed to be lawful, nor are they presumed forbidden, falls under the category of doubtful matters which a Muslim is prohibited from consuming.