On save-for-web, the file extension is not automatically added

From memory the problems started with Catalina and 2019 or 2020, I really can't recall exactly. It feels like forever that such reports as this topic thread have been taking place. I still use Mojave and 2019 for "reliable" work. Scripting would be my suggestion to overcome such issues.

EDIT: Some examples of the issues that I was talking about –

https://community.adobe.com/t5/photoshop/save-for-web-excluding-file-extension-in-output-file/m-p/10767782

https://community.adobe.com/t5/photoshop/file-extension-naming-issues-saving-in-catalina/m-p/10917382

https://community.adobe.com/t5/photoshop/saving-as-copy-instead-of-overwriting/m-p/10691659

https://community.adobe.com/t5/photoshop/file-name-and-extension-highlight-when-saving-on-macos-10-15-catalina/m-p/10738283

Don't shoot the messenger!

There have been what seems like hundreds of people complaining about filenames, overwriting or not overwriting and numerous other issues with Catalina on various Photoshop discussion boards such as this.

Adobe has not fixed the issues with Catalina and guess what, the next macOS is shortly due for release.

Photoshop and later macOS releases after Mojave have been a bit of a trainwreck... But even good old Windows has had issues of late with the 2021 release.

Scripting can work around the issues with Catalina and Photoshop, however, most people just hope and wait for Adobe to correct things (which is not an unreasonable expectation as far as that goes).