I've been experimenting with publishing our layers from Pro (Version 2.2.1), and I noticed that the layers don't appear in the thumbnails when I publish. Only the basemap is visible. This is true whether I try to add a thumbnail image to the the metadata, or just let the layer publish without setting a thumbnail first. They all look like this:
Has anyone else had this issue?
layer thumbnails are stored in the layer's metadata, and if you are sharing a single layer, then it should be there.
Other options don't indicate layer metadata sharing unless it is a single layer.
Create a thumbnail—ArcGIS Pro | ArcGIS Desktop thumbnail info
Introduction to sharing web layers—ArcGIS Pro | ArcGIS Desktop see metadata section
So it is either as designed or it is confusing or related to how you are sharing... which I don't