These brown boxes just showed up for me, not just on OneDrive files, but app icons, shortcuts, etc. on the Desktop.
Not on all, but a lot. I opened OneDrive in Internet Explorer and now all the brown box icons are white boxes with blue arrows pointing to the upper right corner of the white box.
I have no idea why they are there or what they are for. OneDrive has been broken in my Windows install since Win 10 and now I'm on Win 11 and OneDrive tech support could never fix it.
Microsoft, thy name is not Consistency. Sure, let's trust all our important files to the OneDrive Cloud ... I dare you.