In laymans terms, get the devs to change the hardcode.
Horemvores hacky way, edit GlobalDefs and add this:-
<GlobalTriggers>
<OnEvent>OnConquerPlanet</OnEvent>
<Target>
<TargetType>Planet</TargetType>
</Target>
<RandomDurationMin>15</RandomDurationMin>
<RandomDurationMax>15</RandomDurationMax>
<Modifier>
<EffectType>ImmuneToCultureFlip</EffectType>
<Target>
<TargetType>Colony</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>1</Value>
</Modifier>
</GlobalTriggers>
<GlobalTriggers>
<OnEvent>OnPlanetFlippedFromCulture</OnEvent>
<Target>
<TargetType>Planet</TargetType>
</Target>
<RandomDurationMin>15</RandomDurationMin>
<RandomDurationMax>15</RandomDurationMax>
<Modifier>
<EffectType>ImmuneToCultureFlip</EffectType>
<Target>
<TargetType>Colony</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>1</Value>
</Modifier>
</GlobalTriggers>
<GlobalTriggers>
<OnEvent>OnColonizePlanet</OnEvent>
<Target>
<TargetType>Planet</TargetType>
</Target>
<RandomDurationMin>15</RandomDurationMin>
<RandomDurationMax>15</RandomDurationMax>
<Modifier>
<EffectType>ImmuneToCultureFlip</EffectType>
<Target>
<TargetType>Colony</TargetType>
</Target>
<BonusType>Flat</BonusType>
<Value>1</Value>
</Modifier>
</GlobalTriggers>
This was the only solution I could come up with for UCP (It is included with UCP_Core).