fix cmf helper for getPrev

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  |no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

the getPrev / cmf_prev is currently broken. this is more a reminder that we need to further investiate and obviously increase test coverage until this works as intended.