Unable to zoom closer than LOD 18 for xyz raster tile layer

[KirkKuykendall1]

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!

[David_Brooks]

@KirkKuykendall1 , if you add it to map viewer in AGOL, it also restricts you to level 18.

BUT, if you add it to map viewer classic, it lets you go to level 19. Odd!

Maybe add to both save the web maps, open in Pro and then inspect to see what the difference is in the tile request.

[KirkKuykendall1]

@David_Brooks thanks for investigating!

With classic I can zoom down to 20 (at least with chrome Version 93.0.4577.63)

https://arcg.is/0nzaCn

With non-classic I can only zoom to 19 though ... 

https://arcg.is/1SvqbS

When I import either of those maps to Pro, I still end up with max zoom level 18.

Note that QGIS has no problems, and even allows user to set the max zoom level.