Character position desyncing between client and server when walking on Cesium tiles

Cesium for Unreal
1

Hi. I’m working on UE 5.3.2 binary version with the latest version of Cesium from the Marketplace. Whenever I create a new map with a Cesium terrain in multiplayer, I get this strange behavior. When the default Third Person Character walks on the Cesium 3D ,the character’s position in the client does not match the position in the server. Here is a quick test I did where I draw a debug sphere to show where the server thinks the client is positioned:
Cesium-Terrain-Position-Bug
This causes strange movement. You can see that when the user jumps, he gets teleported to the position of the debug sphere, which I think is the server attempting to correct the position of the client. This does not happen in single player.
Upon further investigation, I get this warning:

LogNetPlayerMovement: Warning: ClientAdjustPosition_Implementation could not resolve the new relative movement base actor, ignoring server correction! Client currently at world location X=12.245 Y=-635.460 Z=-91.056 on base https://assets.ion.cesium.com/ap-northeast-1/asset_depot/1/CesiumWorldTerrain/v1.2/17/130979/103042.terrain?v=1.2.0&extensions=octvertexnormals-metadata upsampled L21-X2095665-Y1648679 mesh 0 primitive 0
LogNetPackageMap: Warning: FNetGUIDCache::SupportsObject: CesiumGltfPrimitiveComponent /Game/UEDPIE_1_London.London:PersistentLevel.Cesium3DTileset_0.CesiumGltfComponent_281.https://assets.ion.cesium.com/ap-northeast-1/asset_depot/1/CesiumWorldTerrain/v1.2/17/130979/103042.terrain?v=1.2.0&extensions=octvertexnormals-metadata upsampled L21-X2095665-Y1648679 mesh 0 primitive 0 NOT Supported.

This warning is coming from line 10579 of CharacterMovementComponent.cpp:

// Make sure the base actor exists on this client.
const bool bUnresolvedBase = bHasBase && (NewBase == NULL);
if (bUnresolvedBase)
{
	if (bBaseRelativePosition)
	{
		UE_LOG(LogNetPlayerMovement, Warning, TEXT("ClientAdjustPosition_Implementation could not resolve the new relative movement base actor, ignoring server correction! Client currently at world location %s on base %s"),
			*UpdatedComponent->GetComponentLocation().ToString(), *GetNameSafe(GetMovementBase()));
		return;
	}
	else
	{
		UE_LOG(LogNetPlayerMovement, Verbose, TEXT("ClientAdjustPosition_Implementation could not resolve the new absolute movement base actor, but WILL use the position!"));
	}
}

Here are the steps to replicate this:

  1. Create new project in UE 5.3 with the latest Cesium plugin.
  2. Create a new map. Add a Cesium 3D tileset
  3. Add a custom Cesium Camera on top of the user, pointing downwards. This prevents the user from falling through the floor in multiplayer.
  4. Set Net Mode to Play as Client to start a server with the Editor as a client.
  5. Enter play mode using the default Third Person Character. Walk around and try jumping. The user’s position will abruptly change. I suspect that the server is correcting the client’s position. You will also get the warning message mentioned above.

Strangely, this happens only in newly created maps with Cesium tiles. This does not happen with the existing maps such as those in the Cesium for Unreal Samples in the Marketplace.

2

I’m not sure, but as a first simple thing to check: have you disabled the World Bounds Check?