Is you browser up to date and/or did you try a different browser?
Could be that some of the styling is not available in your browser version.
You can look up the different specific browser capabilities at https://caniuse.com/
Is this a Mx 9.x project upgraded from an older version? As in, is it fully updated to Atlas 3? It might be that you are missing the styling for this widget if you still have Atlas 2 UI resources module in the marketplace modules instead of the Atlas core module.
You could try to rename the Atlas resources module to Atlas core and then download the new Atlas core from the marketplace. Keep in mind this will overwrite custom styling if that is in the resources module. It also may require you to do some work to update your app to the new Atlas 3 styling.