Nginx 1.15.10 – TLSv1.3 Configuration Not Applied

nginx -V
nginx version: nginx/1.15.10
built by gcc 6.3.0 20170516 (Debian 6.3.0-18+deb9u1) 
built with OpenSSL 1.1.0j  20 Nov 2018 (running with OpenSSL 1.1.1b  26 Feb 2019)
TLS SNI support enabled
configure arguments: --prefix=/etc/nginx --sbin-path=/usr/sbin/nginx --modules-path=/usr/lib/nginx/modules --conf-path=/etc/nginx/nginx.conf --error-log-path=/var/log/nginx/error.log --http-log-path=/var/log/nginx/access.log --pid-path=/var/run/nginx.pid --lock-path=/var/run/nginx.lock --http-client-body-temp-path=/var/cache/nginx/client_temp --http-proxy-temp-path=/var/cache/nginx/proxy_temp --http-fastcgi-temp-path=/var/cache/nginx/fastcgi_temp --http-uwsgi-temp-path=/var/cache/nginx/uwsgi_temp --http-scgi-temp-path=/var/cache/nginx/scgi_temp --user=nginx --group=nginx --with-compat --with-file-aio --with-threads --with-http_addition_module --with-http_auth_request_module --with-http_dav_module --with-http_flv_module --with-http_gunzip_module --with-http_gzip_static_module --with-http_mp4_module --with-http_random_index_module --with-http_realip_module --with-http_secure_link_module --with-http_slice_module --with-http_ssl_module --with-http_stub_status_module --with-http_sub_module --with-http_v2_module --with-mail --with-mail_ssl_module --with-stream --with-stream_realip_module --with-stream_ssl_module --with-stream_ssl_preread_module --with-cc-opt='-g -O2 -fdebug-prefix-map=/data/builder/debuild/nginx-1.15.10/debian/debuild-base/nginx-1.15.10=. -fstack-protector-strong -Wformat -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fPIC' --with-ld-opt='-Wl,-z,relro -Wl,-z,now -Wl,--as-needed -pie'

I'm trying to enable TLSv1.3 on Debian Stretch with nginx from official repo of the developers. So far, no dice – Firefox 65 and ssllabs' test don't show it as supporting TLSv1.3, and when I remove the TLSv1.2 support, I get weird "SSL_ERROR_INTERNAL_ERROR_ALERT" fatal error in Firefox. Googling shows no promising leads, and "it works" for everyone (what a shocker!).

Here's the relevant config snippets, all related to SSL/TLS:

listen [::]:443 ssl http2 ipv6only=on;
listen 443 ssl http2;
server_name mysite.intra;

# Use Mozilla's guidelines for SSL/TLS settings
# https://mozilla.github.io/server-side-tls/ssl-config-generator/
# NOTE: some settings below might be redundant
#ssl on;
ssl_certificate /[hidden];
ssl_certificate_key /[hidden];

ssl_session_cache shared:SSL:50m;
ssl_session_timeout 1d;
ssl_session_tickets off;
ssl_prefer_server_ciphers on;
#ssl_protocols TLSv1.3;
ssl_protocols TLSv1.3 TLSv1.2;
ssl_ciphers EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA512:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:ECDH+AESGCM:ECDH+AES256:DH+AESGCM:DH+AES256:RSA+AESGCM:!aNULL:!eNULL:!LOW:!RC4:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS;
ssl_stapling on;
ssl_stapling_verify on;
resolver 8.8.4.4 1.1.1.1 1.0.0.1 8.8.8.8;

And here's the recursive search of configs dir for any mentions of TLS:

Searching 21 files for "TLS"
/mnt/sshfs/mysite/etc/nginx/nginx.conf:
   24      default_type  application/octet-stream;
   25  
   26:     #ssl_protocols TLSv1.2 TLSv1.3;
   27      #ssl_prefer_server_ciphers on;
   28  

/mnt/sshfs/mysite/etc/nginx/sites-available/default:
   21   access_log /var/log/nginx/mysite-access.log combined;
   22  
   23:     # Use Mozilla's guidelines for SSL/TLS settings
   24:     # https://mozilla.github.io/server-side-tls/ssl-config-generator/
   25      # NOTE: some settings below might be redundant
   26   #ssl on;
   ..
   32   ssl_session_tickets off;
   33   ssl_prefer_server_ciphers on;
   34:  ssl_protocols TLSv1.3 TLSv1.2;
   35   ssl_ciphers EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA512:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:ECDH+AESGCM:ECDH+AES256:DH+AESGCM:DH+AES256:RSA+AESGCM:!aNULL:!eNULL:!LOW:!RC4:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS;
   36   ssl_stapling on;

/mnt/sshfs/mysite/etc/nginx/sites-enabled/default:
   21   access_log /var/log/nginx/mysite-access.log combined;
   22  
   23:     # Use Mozilla's guidelines for SSL/TLS settings
   24:     # https://mozilla.github.io/server-side-tls/ssl-config-generator/
   25      # NOTE: some settings below might be redundant
   26   #ssl on;
   ..
   32   ssl_session_tickets off;
   33   ssl_prefer_server_ciphers on;
   34:  ssl_protocols TLSv1.3 TLSv1.2;
   35   ssl_ciphers EECDH+ECDSA+AESGCM:EECDH+aRSA+AESGCM:EECDH+ECDSA+SHA512:EECDH+ECDSA+SHA384:EECDH+ECDSA+SHA256:ECDH+AESGCM:ECDH+AES256:DH+AESGCM:DH+AES256:RSA+AESGCM:!aNULL:!eNULL:!LOW:!RC4:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS;
   36   ssl_stapling on;

10 matches across 3 files

Can anybody please shed the light on what's happenning here?