I cannot install a route named "admin"

It’s a wild situation
I have a slim framework 3 deployed, I’m trying to install a route named admin like this

$app->get("/admin", function ($request, $response, $args) {

But the system redirects me from 301 to 404 error every time. Any other combination of routes, like admn, adm, administration and others, works without interference.
All middleware are disabled, .htaccess has been checked many times, I also looked at the web server settings and so on - there is no reason for 404. I have been trying for hours to come up with at least 1 normal option and start to wonder - maybe this name is reserved inside slim routing? Although, again - why do this?
Guys, I’m desperate. I can use other names, but the very fact that there is some unmanaged element in the system annoys me. Please help me figure it out!