If this still resolves to something old, I'll try to fix it locally, in between other stuff.

Regarding the old OptimizationBase@2.11.0: It seems to be the most recent version that is compatible with MTK@9. My impression is that you'd run into the same problems without this PR when rerunning CI on the backport branch.

Oh, that's unfortunate. Considering this is a minor change and v9 isn't actively maintained, I'll merge this if you confirm that it works for your use case.

I started a downstream CI run with this PR and DifferentiationInterface@0.7.10. I'll report back whether it is successful.

CI passed successfully, without precompilation errors.