I'm able to able to zoom down to level 20 in this map here https://ambergis.com
Here's a level 20 tile: 
However when I add a layer to the map using this:

I can only zoom down to level 18.
I'm unable to add new LODs via the UI:

I saved the layer to an lyrx file, expecting to be able to add two LODs, but don't see any listed.
{
"type" : "CIMTiledServiceLayer",
"name" : "Tiled service layer",
"uRI" : "CIMPATH=map/tiled_service_layer.xml",
"sourceModifiedTime" : {
"type" : "TimeInstant"
},
"useSourceMetadata" : true,
"layerElevation" : {
"type" : "CIMLayerElevationSurface",
"mapElevationID" : "{F68EAF05-3D09-4D83-8AC2-F87B264AFC8A}"
},
"layerType" : "Operational",
"showLegends" : true,
"visibility" : true,
"displayCacheType" : "Permanent",
"maxDisplayCacheAge" : 5,
"showPopups" : true,
"serviceLayerID" : -1,
"refreshRate" : -1,
"refreshRateUnit" : "esriTimeUnitsSeconds",
"blendingMode" : "Alpha",
"allowDrapingOnIntegratedMesh" : true,
"serviceConnection" : {
"type" : "CIMStandardServiceConnection",
"serviceType" : "1",
"url" : "https://ambergis.com/{z}/{x}/{y}.png"
},
"transparentColor" : {
"type" : "CIMRGBColor",
"values" : [
0,
0,
0,
100
]
},
"backgroundColor" : {
"type" : "CIMRGBColor",
"values" : [
0,
0,
0,
100
]
}
}
Is there an undocumented element for CIMTiledServiceLayer that I could insert with my own LODs?
Thanks!