Recent generative AI renders multimodal misinformation structurally harder to detect, making reliable detection dependent on semantic verification grounded in verifiable evidence. However, current benchmarks often fail to isolate true semantic checking from superficial shortcut exploitation. We introduce Falsdo, a diagnostic benchmark designed to make robustness and auditability identifiable. Constructed via a heterogeneous web-mining pipeline, Falsdo provides instruction-conditioned grounding and formalizes a counterfactual artifact-control protocol. This stress test reveals a critical failure: when low-level generation traces are equalized, detector performance degrades, indicating reliance on artifacts rather than robust verification. To enable reproducible diagnosis, we propose DREA, a failure-aligned evidence-auditing baseline. DREA specifies evidence acquisition with explicit noise control and implements constrained channel-wise auditing with reliability-aware late fusion. Experiments demonstrate that Falsdo reliably exposes shortcut dependence, while DREA improves instruction-level grounding (Joint-F1) and minimizes degradation under artifact-controlled settings.