SWF file plays with a small glitch

Hi guys,

I'm having a small but frustrating issue. I've created an animated banner in Animate and the client has asked for the file to be exported as a SWF. However, after exporting and viewing the SWF there is a small but irritating glitch. A small black line appears for a second. While the issue is a minor one, I need to remove it. Has anyone ever had an error like this?