Enable http3 for a subset of domains

[Riccardo Brunetti Host]

Hello.

I have a Nginx server which acts as a reverse proxy for a given number of different domains and I’m trying to enable http3 for a subset of them.

I placed the directives:

>     http3 on;
>     listen xx.xx.xx.xx:443 default_server quic reuseport;
>     quic_retry on;

in the default.conf file and then, only for those domains which I want http3 enabled I also add in the corresponding virtual host:

>         http3 on;
>         listen xx.xx.xx.xx:443 quic;
>         quic_retry on;

In the server section and 

> add_header Alt-Svc 'h3=":8443"; ma=86400';

Inside the location.

Now, what happens is that if I try to open the sites *with* http3 enabled, everything works as expected. If I try to open a site *without* http3 enabled, sometimes I have an SSL error because the server returns the “default virtual host” certificate.
It seems related to the type of browser.

Is it an expected behaviour or I am doing something wrong with the server configuration?

Nginx version: nginx/1.26.1 on ubuntu 22.04

Thanks.
Riccardo
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