Summary

When solving MIP problems with HiGHS and some active solver limits, eg, mip_max_nodes=50, the solver may stop prematurely, ie, before solving the problem to optimality. In that case, the status given by HiGHS is kSolutionLimit. But this is not handled in the if/else chain here:

Hence, the Pyomo user just gets the result.solver:

- Status: unknown
  Termination condition: unknown
  Termination message: TerminationCondition.unknown

Even though, looking at the solver log, we know quite a bit more, eg, that a feasible solution exists:

Solving report
  Status            Solution limit reached
  Primal bound      153.831642438
  Dual bound        154.134587306
  Gap               0.197% (tolerance: 0.01%)
  P-D integral      0.020659109043
  Solution status   feasible
                    153.831642438 (objective)
                    0 (bound viol.)
                    0 (int. viol.)
                    0 (row viol.)
  Timing            0.17 (total)
                    0.00 (presolve)
                    0.00 (solve)
                    0.00 (postsolve)
  Max sub-MIP depth 3
  Nodes             1

Steps to reproduce the issue

I can provide code to generate tricky Knapsack instances, if needed.

The important bit is to use options=dict(mip_max_nodes=1) or similar.

Information on your system

Pyomo version: 6.7.3 (but the relevant code is still the same on main branch)
Python version: 3.12.
Operating system: Ubuntu 24.04
How Pyomo was installed (PyPI, conda, source): PyPI
Solver (if applicable): HiGHS v1.11.0

Additional information

I'm happy to have a go at this in a PR, but might not get to it immediately.