How to check why HTTP/3 is not working for demo Jetty 12.1 server?

I run Jetty 12.1.4 demo according to instruction: https://jetty.org/docs/jetty/12.1/operations-guide/begin/index.html including enabling HTT/3 protocol as described at: https://jetty.org/docs/jetty/12.1/operations-guide/protocols/index.html

On console I see that HTTP/3 started:

...
oejs.AbstractConnector:main: Started oejs.ServerConnector@18b30951{SSL, (ssl, alpn, h2, http/1.1)}{0.0.0.0:8443}
oejs.AbstractConnector:main: Started oejs.ServerConnector@22ee7fdc{HTTP/1.1, (http/1.1)}{0.0.0.0:8080}
oejqqs.AbstractQuicheServerConnectionFactory:main: HTTP/3+QUIC support is experimental and not suited for production use.

netstat shows that Jetty demo is listening on proper ports: TCP 8433 for HTTP/2 and UDP 8444 to which browser should redirect HTTP/3:

mn:~$ netstat -anp | grep 8443
tcp6       0      0 :::8443                 :::*                    LISTEN      38203/java
mn:~$ netstat -anp | grep 38203
tcp6       0      0 :::8443                 :::*                    LISTEN      38203/java
tcp6       0      0 :::8080                 :::*                    LISTEN      38203/java
udp6       0      0 :::8444                 :::*                                38203/java
unix  2      [ ]         STREAM     CONNECTED     189371   38203/java

At first tried with Jetty test self signed certificate. Browsers warned that this certificate is not trusted. Browsers in debug mode (F12) reported that they receive header:

alt-svc: h3=":8443"

but for all pages the only protocol used was HTTP/2.

For google.com I see that HTTP/2 responses redirecting to HTTP/3 used header:

alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000

I changed jetty.sslContext.keyStorePath to point to my own, trusted certificate. I disabled test-keystore module from all demo modules. Now browsers trust my certificate but they still use only HTTP/2 protocol.

Environment:

OS: Fedora release 43 (Forty Three)

Java: # java -version

openjdk version "25.0.1" 2025-10-21
OpenJDK Runtime Environment (Red_Hat-25.0.1.0.8-1) (build 25.0.1+8)
OpenJDK 64-Bit Server VM (Red_Hat-25.0.1.0.8-1) (build 25.0.1+8, mixed mode, sharing)

Browsers: Firefox 145.0 and Chrome 142.0.7444.162

How can I check why HTTP/3 is not used? Is it possible from server side? Is it possible from browser side?

EDIT

After reading my question again, and checking everything with browser and nestat I see that netstat reports UDP 8444:

netstat -anp | grep java | grep 44
tcp6       0  0 :::8443  :::*      LISTEN      40911/java          
udp6       0  0 :::8444  :::*                  40911/java  

but in response headers there is 8443:

alt-svc:    h3=":8443"

Isn't it a mistake? Shouldn't it point to UDP port 8444 as explained at: https://http3-explained.haxx.se/en/h3/h3-altsvc ?

EDIT2

I changed both ports to 433:

[root@stresstest ~]# netstat -anp | grep java
tcp6   0   0 :::443     :::* LISTEN      1661248/java
tcp6   0   0 :::9080    :::* LISTEN      1661248/java
udp6   0   0 :::443     :::*        1661248/java
unix  2      [ ]         STREAM     CONNECTED     66730697 1661248/java

In browser debug I see proper response headers:

alt-svc: h3=":443"

but none of my browser changed protocol from HTTP/2 to HTTP/3.