Page 404 returns code 200

[FlyPastaMonster]

Hello everyone.
We are currently developing an entertainment portal with friends and the frontend has screwed something up so that our page 404 returns code 200, and not 404 as it should.

root@blablabla :/home/user # curl -I https://kapi.bar/404 2>/dev/null | head -n 1 | cut -d$' ' -f2
200

I am trying to prove to him that this is wrong and that it can negatively affect the site's SEO.

Can you tell me if this can really negatively affect the site's SEO or am I overthinking it?

Thanks!