Hello, I have very similar problem. I have already tried everything when printing fonts with the layer effect "outer shadow". Converted to curves, of course, grids and trimmers or just grids. I've written every possible pdf, there are always incorrect results in the print.
There is an angular field around the writing, which is either darkened or has a light background. Everything is displayed correctly in the file, but it is badly incorrect in the print. (There were never any drop shadow issues in Illustrator.)
I've already learned a lot and I'm about to believe that Affinity Designer is useless for printing. Does anyone have an idea how to proceed to rule out such errors? Thanks for an answers
