Billboard height jumps when using CLAMP_TO_GROUND in 1.129 but not in 1.128

[BlockCnFuture]

What happened?

When using heightReference: Cesium.HeightReference.CLAMP_TO_GROUND in BillboardCollection, the billboards' height jumps unexpectedly during zooming in version 1.129. This issue did not exist in version 1.128.

I expected the billboards to stay clamped to the ground smoothly, without jumping in height when zooming the map.

v 1.128：

1.128.mp4

v 1.129:

1.129.mp4

Reproduction steps

1. Add a series of billboards to a BillboardCollection with heightReference set to CLAMP_TO_GROUND
2. Remove all billboards from the collection
3. Re-add the same billboards to the collection
4. Zoom the map in and out
   You will see the billboards jump in height and not remain correctly clamped to the terrain.

Sandcastle example

https://sandcastle.cesium.com/#c=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

Environment

Browser: Microsoft Edge 135.0.3179.98 x64 / Google Chrome 135.0.7049.115 x64
CesiumJS Version: 1.129
Operating System: win 11

[BlockCnFuture]

20250506_184130.mp4

[jjspace]

Thanks for the issue @BlockCnFuture I can confirm I see the billboards jumping around in the latest version but not at 1.128.

I tried a quick git bisect using the sandcastle locally and it seems to be caused by commit b286787 which was part of #12555 but even when I undo that change in QuadtreePrimitive in main I'm still seeing the issue. Might need some more investigation. @ggetz any thoughts?

[BlockCnFuture]

also in 2d mode:

20250507_102525.mp4

[ggetz]

@jjspace Could this have been #12555? That went in shortly after the 1.128 release.

[jjspace]

@ggetz yea that's what I was thinking and git bisect seemed to confirm that but after undoing the change there in the latest main I was still observing the issue. I didn't not dig much deeper than a quick look so maybe I missed something

[BlockCnFuture]

I re-investigated this issue and found that it originates from pull request #12527.
The height caching introduced in #12527 is not correctly updated during small-scale zooming within the scene.

cesium/packages/engine/Source/Scene/QuadtreePrimitive.js

Line 1438 in 52f55d0

const cachedData = tile.getPositionCacheEntry(