Strange desync error in TrueSync's Farseer library

Context:

We added logging to let us know when 'rollbacks' happen. After a rollback, the physics broke - but it only broke in an external build, not the one that was running the editor.

Any ideas on what we should be tracking down?

#RollBack#13.36 Gravity mover velocity rolled back to:-21.6 UnityEngine.DebugLogHandler:Internal_Log(LogType, String, Object) UnityEngine.DebugLogHandler:LogFormat(LogType, Object, String, Object[]) UnityEngine.Logger:LogFormat(LogType, String, Object[]) UnityEngine.Debug:LogFormat(String, Object[]) GravityMover:ApplyUpdate() (at Z:\Projects\Atomech\project-sidescroller\Assets\Scripts\Shared\Movement\Movers.cs:318) MovableUnit:OnSyncedUpdate() (at Z:\Projects\Atomech\project-sidescroller\Assets\Scripts\Shared\Movement\MovableUnit.cs:436) TrueSync.TrueSyncManagedBehaviour:OnSyncedUpdate() TrueSync.TrueSyncManager:OnStepUpdate(List`1) (at Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Unity\TrueSyncManager.cs:781) TrueSync.AbstractLockstep:ExecutePhysicsStep(List`1, Int32) TrueSync.RollbackLockstep:Rollback(Int32, Int32) TrueSync.RollbackLockstep:BeforeStepUpdate(Int32, Int32) TrueSync.AbstractLockstep:Update() TrueSync.TrueSyncManager:FixedUpdate() (at Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Unity\TrueSyncManager.cs:714) (Filename: Z:/Projects/Atomech/project-sidescroller/Assets/Scripts/Shared/Movement/Movers.cs Line: 318) #RollBack#13.36 Rollback from 13.36 UnityEngine.DebugLogHandler:Internal_Log(LogType, String, Object) UnityEngine.DebugLogHandler:LogFormat(LogType, Object, String, Object[]) UnityEngine.Logger:LogFormat(LogType, String, Object[]) UnityEngine.Debug:LogFormat(String, Object[]) AdvancedInputModule:OnSyncedUpdate() (at Z:\Projects\Atomech\project-sidescroller\Assets\NetworkedInputTest\TrueSyncInputSources\AdvancedInputModule.cs:280) TrueSync.TrueSyncManagedBehaviour:OnSyncedUpdate() TrueSync.TrueSyncManager:OnStepUpdate(List`1) (at Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Unity\TrueSyncManager.cs:797) TrueSync.AbstractLockstep:ExecutePhysicsStep(List`1, Int32) TrueSync.RollbackLockstep:Rollback(Int32, Int32) TrueSync.RollbackLockstep:BeforeStepUpdate(Int32, Int32) TrueSync.AbstractLockstep:Update() TrueSync.TrueSyncManager:FixedUpdate() (at Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Unity\TrueSyncManager.cs:714) (Filename: Z:/Projects/Atomech/project-sidescroller/Assets/NetworkedInputTest/TrueSyncInputSources/AdvancedInputModule.cs Line: 280) IndexOutOfRangeException: Array index is out of range. at TrueSync.Physics2D.DynamicTree`1[TrueSync.Physics2D.FixtureProxy].GetFatAABB (Int32 proxyId, TrueSync.Physics2D.AABB& fatAABB) [0x00019] in Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Physics\Farseer\Collision\DynamicTree.cs:285 at TrueSync.Physics2D.DynamicTreeBroadPhase.TestOverlap (Int32 proxyIdA, Int32 proxyIdB) [0x0000a] in Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Physics\Farseer\Collision\DynamicTreeBroadPhase.cs:226 at TrueSync.Physics2D.ContactManager.Collide () [0x00189] in Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Physics\Farseer\Dynamics\ContactManager.cs:374 at TrueSync.Physics2D.World.Step (FP dt) [0x000c2] in Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Physics\Farseer\Dynamics\World.cs:1210 at TrueSync.Physics2DWorldManager.UpdateStep () [0x0000d] in Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Unity\Physics2DWorldManager.cs:70 at TrueSync.AbstractLockstep.ExecutePhysicsStep (System.Collections.Generic.List`1 data, Int32 syncedDataTick) [0x00000] in <filename unknown>:0 at TrueSync.RollbackLockstep.Rollback (Int32 rollbackTick, Int32 temporaryRollbackWindow) [0x00000] in <filename unknown>:0 at TrueSync.RollbackLockstep.BeforeStepUpdate (Int32 syncedDataTick, Int32 referenceTick) [0x00000] in <filename unknown>:0 at TrueSync.AbstractLockstep.Update () [0x00000] in <filename unknown>:0 at TrueSync.TrueSyncManager.FixedUpdate () [0x0006f] in Z:\Projects\Atomech\project-sidescroller\Assets\Standard Assets\TrueSync\Unity\TrueSyncManager.cs:714 (Filename: Z:/Projects/Atomech/project-sidescroller/Assets/Standard Assets/TrueSync/Physics/Farseer/Collision/DynamicTree.cs Line: 285)