3547 frame limit for FxPlugins

When trying to create an effect for 4K video using an FxPlug, I keep running against a 3547 limit in both Motion and Final Cut Pro X. The error message is "This filter has been applied to an object too large to redner, the output will be cropped. The maximum size is 3547x3547".

Which is odd, because obviously Final Cut supports the frame sizes and the plugin itself can support frames of up to 16k pixels wide (it's a hardware-based OpenGL plugin).

Does anyone know where this limit comes from and if there's any way to work around it?