S3D stock setting is probably not going to work with FLUX Delta, you need to double check machine settings.
The 1 layer is normal when importing gcode. It gets converted to fcode as soon as you import it so there is no preview available, FLUX Studio just sees it as flat, or 1 layer. Your normal models that you load in FLUX Studio can render a preview with multiple layers because they are not converted to fcode until you click start and see the "processing" which sends them to the printer. That is when their gcode is getting converted to fcode as well.
But you are saying they do not print. It sounds like you have grabbed the .fff settings file from here but you may not have the machine settings setup right so your gcode isn't actually talking to the FLUX the right way. Check the Gcode Tab in S3D: