Symbols with non-centred anchors drift when zooming

[markfee]

Steps to recreate
In Demo.js, add a symbolAnchor to one or more of historic-monuments sub layers.

Such as [0.1, 0.1], [0.9, 0.9] or [0.5, 0]

Then Zoom the map in and out:
Expected:
The Symbol is anchored to the map, at the relative offset specified in the anchor, but stays 'centred' on that offset position.

Actual:
The symbol's position drifts in the x, y or both axis as you zoom (depending on the offset that is specified).