[BUG] Fix masked targets to respect provided labels

[blenbot]

Reference Issues/PRs

Fixes #599

What does this implement/fix? Explain your changes.

Masked target generation now respects the dataset target labels (y).

Did you add any tests for the change?

No

Any other comments?

No

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files