Glitches of bands of light or dark pixels may appear over images, in transparencies and effects. These may come and go depending on resolution preview mode, zoom and/or if overprint preview is rendered. (The lines are similar to tiling lines in PDFx1a in that they are rendering glitches presumably caused by floating point calculations)

See the thread https://community.adobe.com/t5/indesign/lines-appearing-in-effects-shadow-amp-gradient/m-p/11912229#M419035