CaptainCobbs wrote:

"Isolationist" is an AI personality type that only occurs in a specific few factions

From what I found, it's not as rare as you think (in theory). Thinking about fixing it for all factions within my german language mod, but I'm unsure, if I'd mess it up, as long as I didn't see this bug, yet.

ES2_Localization_Assets_Locales.xml 

1)  <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumSophons">You had one job! Don't trespass with military vessels. You blew it. We're canvassing for Cold War relations ASAP.</LocalizationPair>

2) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumCravers">kkktt--gdo--pftt . . .
FIRMLY: You messed up. Craver territory was off limits for war fleets. Now we want to go back to cold war.</LocalizationPair>

3) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumVampirilis">The Church places great stock on its right to seclusion. Your military trespass tramples roughshod on these desires. You should expect a thawing of relations.</LocalizationPair>

4) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumVenetians">Your military fleets are in sovereign Lumeris waters. This lack of respect isn't to go unpunished. The Families want to return to cold war.</LocalizationPair>

5) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumUnfallen">We had hoped for greater restraint on your part. Once again your military trespasses, and the Heart turns cold.</LocalizationPair>

6) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumTerrans">Tut, tut, tut. You've been caught somewhere you shouldn't have been. Hold out your knuckles. Or whatever you have that passes for knuckles. It's cold war time.</LocalizationPair>

7) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumHoratio">Horatio were spending some time alone so as not to accidentally gaze on imperfection. And you ruined it with your ugly battle fleets crawling into the Horatiocracy. Expect cold war.</LocalizationPair>

8) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumTimeLords">We didn't want visitors. Especially ones with weapons. We now seek a return to more unsympathetic relations.</LocalizationPair>

ES2_Localization_Vaulters_Locales.xml 

9) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumVaulters">Why are your fleets in our territory? It is like waking up and finding Necrophages have invited themselves to breakfast.  You are not ready for a lasting peace.</LocalizationPair>

ES2_Localization_DLC2_Locales.xml 

10) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumMajorHisshos">We do not wish to have your fleets in our territory. I will sharpen my blade and watch them.</LocalizationPair>

ES2_Localization_DLC3_Locales.xml

11) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumUmbralChoir">The Choir watched your clumsy trespass with a mixture of admiration and disgust. We will seek colder relations, Creature.</LocalizationPair>

ES2_Localization_DLC4_Locales.xml

12) <LocalizationPair Name="%DiplomaticInteractionEscalationIsolationistPlayerDisregardedTrespassUltimatumTemplars">You stomp through our space like Endless across the graves of the Lost. Her Majesty wishes you gone!</LocalizationPair>

I've run into this a few times recently.  I hope my limited knowledge of modding will let me fix it.

*EDIT*  Man that was easy to make the changes.