Can't Save. Seeing unusual "Save" icon variant?

This has been happening a lot lately in 2021. 

Open a file, make some changes, go to save and nothing happens. 

When this happens the "Save" icon in the File menu has a yellow warning icon super imposed. 

The only way I can find to work around is to save a copy with a new name, delete original, & rename copy to original.

Why is this happening & how to prevent?

Please advise & thanks!