accepted cultures do not assimilate to the primary, or any other, culture.
If they are non-accepted, then they will assimilate unless:
1) the are in a province that is a core of a country that they are an accepted culture in (for instance, Irish pops in Dublin owned by the UK will not assimilate, but Irish pops in London (not an Irish core), might).
2) They are not affected by the african diaspora modifier. I don't know if this was in vanilla or added by one of the popular mods, but there is a -80 assimilation modifier to african diaspora cultures in north america, that is activated if a country does not have some other modifier like "African acceptance law" or whatever to disable it. Specifically to prevent african Americans from just becoming Yankee, or Afro-Brazilians from becoming just Brazilian, et cetera.
In pic related you can see the section of poptypes.txt (in common) in HFM that prevents afro diaspora cultures from assimilating
-80, more than enough to prevent all assimilation, unless the country has the modifier "african assimilation policy"