Observed

Source-reviewed: enums are among the documented failure modes. Four silent-corruption bugs in the checkpoint serialization layer remain open, all re-verified 2026-06-11: JsonPlusSerializer replaces values whose deserialization fails with None without raising (#6970, opened 2026-02-28); StrEnum fields coerce to plain str on round-trip (#6598, opened 2025-12-16); nested Enum fields become None (#6718, opened 2026-01-24, closed once and reopened); BinaryOperatorAggregate stores the Overwrite wrapper instead of the payload when the initial value is MISSING (#6909, opened 2026-02-23). Checkpoint round-trips are lossy for non-primitive types with no exception raised.

1. UPDATE 2026-06-28(re-verified on 1.2.6): all four serialization bugs remain open and the failure class is expanding — #8184 (dict subclasses lose type fidelity) and #8185 (Fraction/complex rejected with TypeError) were filed 2026-06-24; the StrEnum fix PR #6602 is open but a maintainer flagged it may not change behavior.
2. UPDATE 2026-07-17(re-verified against 1.2.9, released 2026-07-10): all four original serialization bugs (#6970, #6598, #6718, #6909) and the two newer ones (#8184, #8185) remain open; #6602 (StrEnum fix) is still open and unmerged. 1.2.7/1.2.8/1.2.9 shipped no fix for any of these — the only Overwrite-related fix in this window (#8127, merged 1.2.7) addresses a different code path (JSON-boundary roundtripping) and does not touch #6909's MISSING-fast-path bug.