Similar (but not exactly the same) problem here: Text was set in c0, m0, y0, k100. When I open the resulting PDF, Affinity Designer (and PhotoLine as well, for that matter) still shows the right values for the objects. The guy from the printer service however says it’s a “deep black” (with c, m, y NOT being zero). No idea what’s going wrong. Screenshot of export settings is attached – sorry it’s in German. Affinity Designer 1.7.3 on macOS Mojave.