Exporting SWF with masks

My After Effects compositors love when I export my animation in SWF format. It makes them very happy.
They say things like, “Wow, you sure you’re not using Animate CC?” and “That’s super cool that Harmony can easily export SWF’s for us to use in After Effects!”
But it’s not always sunshine and lollypops.
It seems that if I use masks in my project, they are not respected in the SWF export. I can export my animation, but the masked out areas appear there, and look weird.
Not the final image that I expect there to be.
Anyone know what’s going on? Or can I simply not output SWF format if I’m using masks in my project?