Some verbs have dual forms for the participio passato, for example retrocedere (retrocesso, retroceduto). In this case WordReference (https://www.wordreference.com/conj/ItVerbs.aspx?v=retrocedere) lists both forms. But for the verb propendere (https://www.wordreference.com/conj/ItVerbs.aspx?v=propendere) it gives only a single form: propeso. Whereas Educalingo (https://educalingo.com/it/dic-it/propendere) gives only the single form propenduto. If the two forms are both commonly used, why does WR give only one? If one is much more common than the other, which is it?
The book Italiano by Serianni says (XI.280) that the normal form is propenso (not *propeso), while propenduto exists but is uncommon. This certainly confirms my experience as a native speaker (I do not think I ever heard or read the form propenduto).
As an aside, while retroceduto does not feel "wrong" (probably by the influence of the by now fully regularized cedere), I would still consider much more common the form retrocesso.