403 Errors in 2D (Leaflet) mode

[geoffrowland]

I'm getting 403 errors with the 2D (Leaflet) mode of TerriaMap with some Url-Template sources..No problemm with the 3D Terrain or 3D Smooth (Cesium) modes.
This results in the display of missing image borders and broken image icons in the browser
This is despite having "treat403AsError": false, "treat404AsError": false, "ignoreUnknownTileErrors": true in the .json config file.
In some cases, the problem can be avoided by setting an appropriate clipToRectangle, or serving up transparent tiles, but this is not always possible with 3rd-party URL-Template sources.
It may result from changes in browser behaviour in recent updates. A while back. I noticed the issue with Chrome, but not Firefox. Now both browsers are impacted. I have not found any browser settings to fix the problem
I don't get the same issue with Leaflet (not integrated in TerriaMap). So, I presume this reflects differences in the missing-tile/error-handling between TerriaMap and Leaflet. Perhaps in the promise-handling in terriajs/lib/Map/CesiumTileLayer.js?
Any insight or fix gratefully received.