diff-bundle reports too many false positives

I would expect for the following to report no differences:

$ juju export-bundle > exported-bundle.yaml
$ juju diff-bundle ./exported-bundle.yaml

The output is however full of false positives, which makes diff-bundle kind of unusable.

Here are a few issue classes I have identified:

* Relation ordering:

relations:
  bundle-additions:
  - - ceilometer-agent:ceilometer-service
    - ceilometer:ceilometer-service
  model-additions:
  - - ceilometer:ceilometer-service
    - ceilometer-agent:ceilometer-service

* Incorrect parsing of machine/charm series:

applications:
  aodh:
    series:
      bundle: ""
      model: xenial
machines:
  "0":
    series:
      bundle: ""
      model: xenial

(the exported bundle only specifies 'series: xenial' at the top)

* Incorrect handling of user-provided values that are equal to charm defaults:

  aodh:
    options:
      haproxy-client-timeout:
        bundle: 90000
        model: 90000

(90000 is explicitly set, but is equal to the charm default)

* Over-exporting of default values (which is more like a bug in export-bundle, I suppose):

  ceph-mon:
    options:
      monitor-count:
        bundle: 3
        model: null

(monitor-count is not set, but appears in the exported bundle nonetheless)