Hey, most the files with white the rip doesn't recognize unless its a slightly off-white or grey so it could be the overprint black ink because I know when I want white highlights to print in my photoshop files I have to slightly change the absolute white to be slightly grey. So maybe check and see if that may be something involved.
Also check your silicon/parchment sheet(s) to make sure your not pressing a dirty sheet onto the shirts especially since when you take the sheet out to put the shirt in or take the shirt out the teflon/parchment sheets will collect some dust easily especially if static etc. So it could be the dust or residue from the table etc.
Clean the sheet or try a new one and makesure the rubber from the bottom platen of the press isn't wearing etc onto the heater element of the press or sheet of silicon/parchment etc.
I know when curing white ink with overprint color etc your supposed to lightly pressure or hover it. I do little to no pressure and always use parchment paper for final curing as I don't like the way the teflon/silicon sheet cures the ink to be glossy it just looks tacky to me and I enjoy the matte look the parchment paper provides.
If you can I would upload images of the artwork printed, pre press, post press, and maybe of the shirt pre and post press/cure with the pretreat.
Also remember when using any polyester there's a process that happens under high curing temps called Dye Migration so on a black shirt the white ink would turn light grey or even dark grey just from the dye in the fabric. I know with screen printing using white ink this happens very badly like a yellow shirt with a 50/50 blend white ink will turn yellow or orange etc so I cure to temp and no higher otherwise the migration will start in.