I'm testing migration of PEX
I see the following:
PEX:

  rank-a:
    options:
      default: true
      prefix: '&a'
    permissions:
    - blah

Is being translated into LP like:
default.yml
(empty file)
rank-a.yml

name: rank-a
permissions:
- blah
meta:
- default:
    value: 'true'

I wonder why LP would not prefer to make a default.yml, place the rank-a permissions into this one and make this default rank have a displayname "rank-a"

I think this will make migration a lot more simple for people just starting with LP, like me ;-)
If this is intentional, I would like to learn the strategy behind it.

LP just takes the meta values from pex. Some might be not used then (like the meta default)

However, this is intentional since PEX allows you to set multiple groups as default.
And since LP only really has 1 default, this would cause issues.

Technically it may be possible to have multiple default groups in PEX, but practically, seriously, I would not know why anyone would like to have that.

Indeed you're right - however I want to keep the migration script fairly simple. The transition of the default group should be a manual process imo - as there are a number of approaches to handling default groups in LP.