Comment 6 for bug 2030349

Multipass is currently not configuring the fqdn correctly.

hostname --long should return microstack.multipass but does not.

As workaround you can create a cloud init file with:
#cloud-config
fqdn: microstack.multipass

And run:
multipass launch --name microstack --cloud-init cloud-init.yaml --cpus 4 --memory 16G --disk 50G && multipass shell microstack

I've created an issue on multipass repository, and I will follow up depending on the progress: https://github.com/canonical/multipass/issues/3277