Data connections are recognizing .asc files as text files not Rasters. Add to Map is not available nor can they be added using the Add Data dialog. Dragging the .asc files using Windows File Explorer works!
plus 8 more from 2015 to 2018
I suspect being recognized as a text file is the symptom of the issue since you can do the drag thing
Oddly enough I can drag them from the Catalog pane to a tool. I tried the Raster to Other Format tool for example.
I cannot add the .asc file to a map by right clicking or by dragging to the map canvas.