Ansible 2.2: register with a variable in the name

Greetings

Since moving from Ansible 2.1.3 to Ansible 2.2.0 I have run into an
issue with register, and when it's variable name is built up using a
variable. I have no idea if this is due to a regression in Ansible 2.2,
or if I have been doing something wrong/unsupported which I finally got
bitten by.

Anyway, this being what breaks between 2.1.3 and 2.2.0. Context wise
it's part of a parameterize role.

  - name: Add {{ repo_name }} sources
template:
src={{ repo_name }}.list.j2
dest=/etc/apt/sources.list.d/{{ repo_name }}.list
owner=root
group=root
mode=0444
register: reg_{{ repo_name }}_sources
  
  - name: debug reg_{{ repo_name }}_sources
debug: var=reg_{{ repo_name }}_sources

Running Ansible 2.1.3 I get the following expected output.

  TASK [repo : Add gitppa sources] ***********************************************
  ok: [vagmast]
  
  TASK [repo : debug reg_gitppa_sources] *****************************************
  ok: [vagmast] => {
"reg_gitppa_sources": {
"changed": false,
"diff": {
"after": {
"path": "/etc/apt/sources.list.d/gitppa.list"
},
"before": {
"path": "/etc/apt/sources.list.d/gitppa.list"
}
},
   "gid": 0,
"group": "root",
"mode": "0444",
"owner": "root",
"path": "/etc/apt/sources.list.d/gitppa.list",
"size": 61,
"state": "file",
"uid": 0
}
  }

While with Ansible 2.2 that register never appear to happen.

  TASK [repo : Add gitppa sources] ***********************************************
  ok: [vagmast]
  
  TASK [repo : debug reg_gitppa_sources] *****************************************
  ok: [vagmast] => {
   "reg_gitppa_sources": "VARIABLE IS NOT DEFINED!"
  }

Any idea what I'm doing wrong, or what I should try?

// Andreas

Ok, here is a minimal playbook showing my particular issue.