HELP PLEASE! White background appears as gradient when running prototype

Hi everyone,
I'm having an issue with my Figma prototype. One of the white rectangles in my design — which has a plain white "glacé" effect but no opacity or gradient — suddenly appears with a gradient when I run the prototype.
I've double-checked the fill settings, and it's set to solid white (#FFFFFF), with 100% opacity and no effects applied.
Has anyone experienced this before? Could it be a rendering bug in prototype mode, or am I missing something?

Thanks in advance!