Ensure that under Layout Properties (if yuo don't see a tab in the Map Layout window, go to Layout > Layout Properties), that 'Print to raster' is unchecked.
Sometimes it also helps to make sure 'Always export as vectors' is checked.
If you have certain layer effects enabled (transparency, blending modes) it may come up with a warning message saying ...
Printing layers using clustering will create a client-side image of the layer in the printout.
There is no current support for printing legend items for layers that are sent as a client-...
It seems there is a bug, or maybe intentional behavior where the print widget just sends your layer as a raster layer to the printing server.
The map JSON it is sending is looking at your layer as an image.
It is sending the image in base64: