Singularity connection differences between 'instance start' and 'run' with custom network portmap and ssl self-signed certificate

Question

I have built a singularity container from the docker hub registry like so:

sudo singularity build \
    postgres12.sif \
    docker://postgres:12

And can successfully run the container like:

sudo singularity run \
    -B postgres12_data:/var/lib/postgresql/data \
    -B postgres12_run:/var/run/postgresql \
    --net \
    --network-args "portmap=9932:5432/tcp" \
    postgres12.sif \
    -c ssl=on \
    -c ssl_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem \
    -c ssl_key_file=/etc/ssl/private/ssl-cert-snakeoil.key

It requires starting with root, which I don't love, but subsequent connection is available with:

psql "sslmode=require" -h 0.0.0.0 -p 9932 -U postgres
Password for user postgres: 
psql (13.5 (Debian 13.5-0+deb11u1), server 12.10 (Debian 12.10-1.pgdg110+1))
SSL connection (protocol: TLSv1.3, cipher: TLS_AES_256_GCM_SHA384, bits: 256, compression: off)
Type "help" for help.

postgres-# \l
                                 List of databases
   Name    |  Owner   | Encoding |  Collate   |   Ctype    |   Access privileges   
-----------+----------+----------+------------+------------+-----------------------
 postgres  | postgres | UTF8     | en_US.utf8 | en_US.utf8 | 

So far, so good.

However, when I try to start this as an instance, e.g.:

sudo singularity instance start \
    -B postgres12_data:/var/lib/postgresql/data \
    -B postgres12_run:/var/run/postgresql \
    --net \
    --network-args "portmap=9932:5432/tcp" \
    postgres12.sif \
    postgres-ssl-01 \
    -c ssl=on \
    -c ssl_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem \
    -c ssl_key_file=/etc/ssl/private/ssl-cert-snakeoil.key

Then I cannot access the postgres service. I can see the instance, however:

sudo singularity instance list
INSTANCE NAME      PID      IP            IMAGE
postgres-ssl-01    12345    [IP Adresss]  /PATH/TO/postgres12.sif

Can anyone offer any insight into what I'm doing wrong? Thank you!