[Backport] Fix faulty validation for maintained forward model objectives.

[DanSava]

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'pytest tests/ert/unit_tests -n logical -m "not integration_test"')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Create Backport PR to latest release

[eivindjahren]

👍

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.80%. Comparing base (eff341e) to head (a51afc7).

Additional details and impacted files

@@               Coverage Diff                @@
##           version-11.1    #9225      +/-   ##
================================================
- Coverage         90.85%   90.80%   -0.06%     
================================================
  Files               352      352              
  Lines             21959    21959              
================================================
- Hits              19951    19940      -11     
- Misses             2008     2019      +11     

Flag	Coverage Δ
cli-tests	39.20% <ø> (-0.02%)	⬇️
gui-tests	71.81% <ø> (-0.01%)	⬇️
performance-tests	49.33% <ø> (-0.04%)	⬇️
unit-tests	79.63% <ø> (-0.03%)	⬇️

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