Ah, I should have said so.

The mission in question is "French Civilization" under the debug name "UKS_cultures_of_France"

Alright, found a workaround until devs fix it.

I imported code from the Vanilla British mission tree to do this.
You wanna type this into ME_England_Decisions.txt under EU4/decisions folder:

eng_change_anglois = {
effect = {
custom_tooltip = ave_promote_angevin_culture_bt
hidden_effect = {
change_primary_culture = anglois
set_ruler_culture = anglois
if = {
limit = { has_heir = yes }
set_heir_culture = anglois
}
if = {
limit = { has_consort = yes }
set_consort_culture = anglois
}
every_subject_country = {
limit = {
culture_group = british
NOT = { primary_culture = american }
}
change_primary_culture = anglois
}
every_province = {
limit = {
culture_group = british
owner = {
OR = {
tag = ROOT
is_subject_of = ROOT
}
}
}
change_culture = anglois
}
}
}
ai_weight = {
factor = 0
}

I didnt put a lot of effort into it, make sure to delete it if you arent playing England.

20 years ago doesn't seem to be enough time. I cannot reasonably conclude that it doesn't work from this alone