Reported by Discord user Skummeh:  

https://discord.com/channels/227126593754103810/230330494120755200/1215516739312418827

Issue is a TemporaryEffect granted to host that isn't transferred to the other player, causing desync.

  <TemporaryEffectDefinition Name="OnPirateBaseDestroyed" Category="AcademyRelationWithMajor" Duration="20" Hidden="true">

     <Path>ClassEmpire</Path>

     <SimulationDescriptorReference Name="TemporaryEffectOnPirateBaseDestroyed"/>

  </TemporaryEffectDefinition>

A temporary workaround is nulling out that effect to 0 turn duration with no attached benefits.

  <TemporaryEffectDefinition Name="OnPirateBaseDestroyed" Category="AcademyRelationWithMajor">

  </TemporaryEffectDefinition>

User Ninakoru has included the workaround in the Community Fixes mod just now. But a more permanent fix will be needed for everyone that plays without mods.

https://steamcommunity.com/sharedfiles/filedetails/?id=3168837637

Issue seen by me too, same delta in dump files