conda/conda-build

Hi everyone, I may have mentioned this problem before, but while I did get it to work briefly, I do not belief I found a proper solution. My problem is related to build variants in combination with multiple outputs in a recipe.
As an example, I created a minimal example recipe:

package:
        name: outputvariants
        version: "0.1"

build:
        number: 0

requirements:
        run:
                - out_a
                - out_b

outputs:
        - name: out_a
          script: install_A.sh        # [unix]
          requirements:
                  build:
                        - {{ compiler('c') }}
                        - {{ compiler('cxx') }}
                        - cmake >=3.10
                  host:
                        - boost-cpp {{ boostcpp }}
                  run:
                        - boost-cpp {{ boostcpp }}

        - name: out_b
          noarch: python
          script: install_B.sh
          requirements:
                  host:
                          - python
                  run:
                          - python

with minimal build scripts that only do `touch out[A|B]file.
If i setup my conda_build_config.yaml with at most 2 variants, e.g. as such:

boostcpp: 
   - 1.68
   - 1.69

it builds as expected, but if I add more boost variants, or something else, liek so:

boostcpp: 
   - 1.68
   - 1.69
   - 1.70
   - 1.71
python:
   - 3.6
   - 3.7
   - 3.8

I only get a IndexError:

...
File "/Users/jpoeppel/miniconda3/lib/python3.7/site-packages/conda_build/metadata.py", line 2173, in _get_used_vars_meta_yaml_helper
    apply_selectors=apply_selectors))
  File "/Users/jpoeppel/miniconda3/lib/python3.7/site-packages/conda_build/metadata.py", line 1698, in extract_single_output_text
    output = output_matches[output_index] if output_matches else ''
IndexError: list index out of range

Since I cannot imaging being the only one trying to have multiple outputs build with variants, I must be doing something wrong, but I do not quite get what the problem is. I would be happy about any help!