Root Cause

The failing assertion is in jstests/noPassthrough/query/joins/join_optimization_server_status.js:

assert.docEq({}, fallbackReasonDelta(before, after), "query was fully optimized", attr);

The joinOptimization serverStatus metrics (including fallbackReasons) are cumulative counters for the whole mongod process, not scoped to the test's own query. FTDC periodically issues its own internal aggregate commands (e.g. a $collStats pipeline) against the server. Those pipelines never contain a $lookup, so they pass through AggJoinModel::pipelineEligibleForJoinReordering() and increment the shared fallbackReasons.noLookup counter, just like any other non-join pipeline would.

When an FTDC collection cycle happens to fire in the narrow window between the test's "before" and "after" serverStatus() snapshots, it increments noLookup by 1 even though the test's own $lookup pipeline was fully optimized. This causes the assert.docEq({}, ...) check to intermittently fail with:

expected document { } and actual document { "noLookup" : 1 } are not equal : query was fully optimized

This is a test-isolation issue, not a join optimizer bug: every per-query counter/histogram for the test's actual pipeline (joinOptimizationConsidered, joinOptimizedTrue, numEnumerations, and all histogram deltas) matched exactly in the failing run, confirming the query itself was optimized correctly.