Hello - I can't believe I haven't run into this until now, but it appears that any layer with an Arcade expression defined / used for the symbology will not print. That is, the layer is not output as part of the map export when using the default AGOL or Enterprise 10.7.1 print service, from either map viewer classic, map viewer beta, or the Web Application Builder print widget
My guess is the user-defined Arcade field name used for display is not recognized by the print service, regardless of where the layer is consumed.
Edit - This appears to be a problem with large polygon layers (like a county wide zoning) with an Arcade defined symbology defined. I did just successfully print a transit-stop layer with some 1500 points that has an Arcade defined expression.