Why are you running mix tailwind default --watch though? All you should need to do is run mix phx.server.
EDIT: I just tried a new app and tried running mix phx.server and mix tailwind default --watch and I still wasn’t able to reproduce it so I’m really not sure what could be wrong 
Yes, I know. I still get the bug when I run mix phx.server. I’m just using mix tailwind default --watch to illustrate the point, as it’s the same command that mix phx.server will run under the hood to compile your tailwind files.
The issue mentioned above should have been fixed in the latest tailwindcss. A newly generated project using latest mix phx.new should work fine.
You can try to run the tailwind cli directly like tailwindcss -i ./src/input.css -o ./src/output.css --watch (more detailed instructions on Installing Tailwind CSS with Vite - Tailwind CSS ) to check out if this is a tailwind upstream issue.