If you want to keep it vectorised, I’ve found flattening PDF’s in acrobat used to solve that for me. Takes about a minute but easy to set up an action.
make a watermark over image, consisting of a couple spaces “ “ as a placeholder (you want the watermark to remain invisible, you can put anything you want in that box though.
drop watermark opacity to 0%.
open up flattener preview, then convert all text to outlines (I can’t remember what else I usually tick but I think it’s pretty much the default settings).
We only used to get that problem in rasterlink, haven’t had it in onyx yet. Though I do remember you could see transparency issues in the print preview.