api: fix custom coeffs with cross derivatives

[mloubout]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.30%. Comparing base (aed4fa6) to head (3ad8470).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2672      +/-   ##
==========================================
- Coverage   91.99%   91.30%   -0.69%     
==========================================
  Files         245      245              
  Lines       48880    48906      +26     
  Branches     4314     4313       -1     
==========================================
- Hits        44966    44655     -311     
- Misses       3217     3537     +320     
- Partials      697      714      +17     

Flag	Coverage Δ
pytest-gpu-aomp-amdgpuX	72.53% <28.57%> (-0.01%)	⬇️
pytest-gpu-nvc-nvidiaX	?

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