hmmm.  This would be exceedingly odd.  The only way I could see Expulsion having results like that from the entity files would be if the new culture entity modifer was bugged and the buff effect from expulsion were reducing culture ouput insteado f increasing it.

This could warrant some testing.

TAX INCOME buff is right;

Only the right planet can get +20% tax; The enemy ship including tradeship etc.

The culture spread is bugged.

I've found this bug just two days ago, and fix it like this:

Open your BuffExpulsion.entity

Replace "CultureSpreadModifier" with "CultureSpreadRateForOwner"

Then it's fixed.

Here is a fixed binary file made for you, just put and overwrite the original one.

Dropbox Download

I find that your own trade ships leaving your gravity well gives you culture and tax increase as well, quite odd.