Try removing the Downloads folder from the Dock and adding it again. Click and drag the folder up and away from the Dock until the work 'Remove' appears and release it.
Then, open a Finder window, navigate to your Home folder and drag the Downloads folder from there back to its proper location in the Dock.
Use folders in the Dock on Mac - Apple Support
If that doesn't work and a simple reboot of your Mac doesn’t fix things, see if the problem still happens in safe mode. Safe mode will often correct weird software behavior by forcing the OS to clear caches, do disk repair and other self-maintenance and housekeeping and temporarily disables third-party mods.
How to use safe mode on your Mac - Apple Support
You can exit safe mode by restarting your Mac normally, then reevaluate the problem.