Why can a Java stream count skip the work placed in peek?

An observation hook is not a reliable place for required processing. A terminal operation may determine its answer without evaluating every intermediate action. Move required validation or persistence into an explicit operation whose result is consumed, or use a straightforward loop when the purpose is side effects. Keep peek for diagnostics that are not necessary for correctness. Adding a meaningless filter merely to force traversal hides the design problem and can become fragile after another change. Verify with a sized list, a filtered stream and an empty input. Assert the required output or persisted effect rather than a logging call count. A stream implementation may optimize legitimate operations differently, and parallel traversal introduces ordering constraints of its own.