I also noticed that if I use PortId::default() as a port, I get the following error: could not retrieve module from port-id: ports/defaultPort: capability not found. Is this expected?

Could you give more details on the use that shows this error?
But in any case, we should probably remove or change to transfer.

I also noticed that if I use PortId::default() as a port, I get the following error: could not retrieve module from port-id: ports/defaultPort: capability not found. Is this expected?

Could you give more details on the use that shows this error?

The error should show up with something like this:

    let connection_0_1: Connection = ...;
    let ordering = Order::default();
    let port_0 = PortId::default();
    let port_1 = PortId::default();
    let channel_0_1 = Channel::new(connection_0_1, ordering, port_0, port_1)?;

The full example that triggered the error is here: https://github.com/vitorenesduarte/ibc-rs/blob/reproduce_bug_rust/bbt/src/main.rs#L74-L80 (the branch is not polished; I can extract the example from there in case it's useful)

I just confirmed, and the error can also be triggered using the CLI by replacing transfer with defaultPort in the following:

rrly -c loop_config.toml tx raw chan-init ibc-0 ibc-1 connection-0 transfer transfer defaultChannel defaultChannel

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just confirmed, and the error can also be triggered using the CLI by replacing transfer with defaultPort in the following...

Yes, that is a chain error and it is expected as there is no module on chain binding to that defaultPort. The only supported one is transfer. The relayer does not have currently a way to query valid PortId -s so if the ID is valid it just passes it in the IBC message.