This is related to SEF Urls.

Disable SEF Urls and it works fine.

Therefore I'll offer up my nginx configuration (as per #29303) of

## Enable SEF URLs
# Joomla API application
location /api/ {
	try_files $uri $uri/ /api/index.php?\$args;
}
# Joomla public frontend application
location / {
	try_files $uri $uri/ /index.php?\$args;
}
## -- End

Tested with official Joomla 3 docker images (APACHE) (using Joomla 4.0-dev code) and it works as expected with and without SEF and with and without .htaccess, so this is a nginx specific issue...

Finally got to the bottom of this

ok this problem is with the nginx configuration provided by @nikosdion in #29303 - sorry Nic :)

When you have

/index.php?\$args

in your try_files in nginx location blocks and then go to a url of

http://127.0.0.1/index.php/search?q=blog

you get an Input object of

as soon as I remove the \ I get the expected result input of the below and search works again.

Therefore the problem is that the advice for nginx users in #29303 is slightly wrong :)

Also going to post https://docs.joomla.org/Nginx here, as that documents nginx config with Joomla, and has no \ before the $args :)

Closing as nothing to "fix" per se because ultimately its just a typo in the advice given in #29303, if Nic can update that then people (like me) will not copy and paste the wrong thing again :)

@PhilETaylor Thanks for the heads up! I had copied the code from Admin Tools... forgetting that since it was a PHP heredoc I had escaped dollar signs. I updated the comment in #29303 by removing the backslashes. Can you please give it another try?

Without the slashes it works as expected and search works again as per the debug in #29985 (comment)

Thanks Nik - I just did not want others to fall over the same issue I had :)

No problem. I totally fat fingered that copy and paste ?

Could be worse, it could have been in the htaccess.txt and not in some Nginx config that not many people will use ;-)

/me goes hunting in the htaccess.txt next hahahaha