When Layer FX's blend mode is set to other than Normal, it will be incorrectly rasterized on PDF/X-4 export.
PDF/X-1a has no problem.
Also I would like you to make exporting Outline FX as a separate element possible.
Some FX can be exported as a separate element such as Outer Glow or Outer Shadow.
Here's the FX on the document
Top Left: Outer Shadow 50% / Normal, Top Right: Outer Shadow 50% / Darken
Bottom Left: Outer Shadow 100% / Normal, Bottom Right: Outer Shadow 100% / Darken
PDF/X-4
PDF/X-1a
af-fx-test.afdesign