Finder automatically changing file extension to "processed"

I'm running Big Sur on my M1 Mac Mini, and I'm running into a Finder problem that is perplexing me.

In my Dropbox folder, subfolder Downloads, Finder is automatically appending a file extension of "processed" to my files. It does not happen immediately, but the problem eventually seems to rename all files to include the "processed" extension. For example, unnamed.png will inevitably become unnamed.png.processed.

In my troubleshooting, the issue seems to only affect this folder, no others. I have tried force-quitting Dropbox and the Dropbox Finder Extension, and the problem continues to persist.

I've done my fair share of searching, but I cannot find other users dealing with this issue. Does anyone have any ideas to diagnose and resolve the problem?