There is some kind of (longstanding, see below) typo or copy&paste error in Jipopt::finalize_solution that leads to the objective function value not being copied correctly: the code (at src/Interfaces/IpStdJInterface.cpp:668) reads:

   env->GetNumberArrayRegion(fj, 0, 1, &obj_value);

where it should read:

   env->SetNumberArrayRegion(fj, 0, 1, &obj_value);

(note Set, not Get).

This has been broken since basically forever. At DAGOPT, we had carried a local workaround for that in our wrapper classes since 2017 (when we first noticed the issue, but had not yet found the bug in the code), and a local patch for Ipopt 3.12.13 since 2019 (when I realized the fix was literally one character). Unfortunately, it looks like I had somehow forgotten to report this upstream, and it looks like nobody else noticed it either, because it is still broken in the current 3.14.17 release and the current stable/3.14 branch. So I am reporting it now. Better than never, I guess.

Note that the code in 3.12.x was using GetDoubleArrayRegion, this was since updated to use the GetNumberArrayRegion macro, but the wrong "Get" that should be "Set" was unfortunately not noticed when that change was made either.

GetNumberArrayRegion (i.e., GetDoubleArrayRegion) cannot possibly be the right method to use here, because it copies the current value of fj to the by-value argument obj_value of Jipopt::finalize_solution that is discarded immediately thereafter when the method returns. Changing it to SetNumberArrayRegion should do the right thing (copying from the obj_value argument to fj, as for all the other arguments) and be an obvious typo fix. (I can definitely confirm it fixed the issue in 3.12.13. I have not yet tested it in the current 3.14.17 because I am working right now on porting our build scripts, which is how I have noticed that the bug is still present.)