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Looks like it was intended because the mission file for all the missions that add development on completion of the mission say something like, using the unite Tigray mission as an example...
effect = {
tigray_area = {
limit = { owned_by = ROOT }
add_base_production = 1
add_province_modifier = {
name = "accomodate_tribal_leaders"
duration = 9125 #25 years
}
}
The key line being limit = {owned_by = ROOT}
Which means for it to just apply it to your direct owned lands
If you want the dev boost to apply to your vassals lands in those areas when you complete the mission you'd have to go into the mission file itself and either remove that line completely or comment it out with a # at that beginning of that line and apply to all instances.