Ran into it again,  very reliably reproduced,  and this time I thought to do a "before" save (attached).  V1.5.49 S5 GOLD-Public.  No mods,  all DLC.

To reproduce:

1. Load save.   Note that terraforming Ainae V costs 560 production,  which is exactly 50% off thanks to World Architects.

2. End Turn

3. Save to "temp" or some other name (don't overwrite the original)

4. Quit to main menu,  or all the way out to the OS,  or do nothing.  All three produce the same result.

5. Load "temp".   Note that terraforming Ainae V costs 112 production,  or 90% off thanks to World Architects being doubled up.

I understand if this is too "obscure" to be fixed,  but it has game-breaking potential anytime the Sowers show up.

Alternately,  is there a way to remove Sowers from the list of potential minor civs?  I could live with that as a personal workaround.

Thank you!

Sower Terraforming Bug.sav

Also note, I don't see anything wrong with the XML (not that I'm an expert).  

from Simulation/SimulationDescriptors[FactionTrait_Update8].xml

<SimulationDescriptor Name="FactionTraitMinorWorldArchitects" Type="FactionTraitMinor">
<Modifier TargetProperty="TerraformationModifier" Operation="Percent" Value="-0.5" Path="ClassEmpire"/>
<Modifier TargetProperty="MinorTraitsAssimilated" Operation="Addition" Value="1" Path="ClassEmpire" TooltipHidden="true"/>
</SimulationDescriptor>

from Simulation/MinorFactionPersonalityDefinitions[Update8].xml:

<MinorFactionPersonalityDefinition Name="FactionPersonalitySowers">
<MajorTrait Name="PersonalityTraitPragmatist"/>
<MinorTrait Weight="50">
<Trait Name="FactionTraitMinorSoilHealers"/>
</MinorTrait>
<MinorTrait Weight="35">
<Trait Name="FactionTraitMinorWorldArchitects"/>
</MinorTrait>
</MinorFactionPersonalityDefinition>

The only "different" thing here is that the Sowers aren't part of the main XML, they're in [Update8] but if that was really an issue I imagine there would be a lot of other things that didn't work.

The only other thing I can think of is that,  in this latest game,  I had the assimilation quest active when I assimilated the Sowers via influence.   I wonder if that somehow triggered the double-assimilation?   If so this could be a more generic bug with minor faction assimilation... should be easy to test,  I'll see if I can repro with a different minor faction.