X_FLAG_EXPIRY_INVALID
Means
a temporary flag has no usable expiry date
Typical cause
expiresAt absent, or not an ISO-8601 date or zoned date-time naming a real day ('December 1, 2026', '2026-02-30', '2026-12-01T00:00:00') — reachable from a store snapshot or plain JS; in TypeScript the union already refuses it
Fix
set expiresAt to an ISO-8601 date such as '2026-12-01' in defineFlag({ key: '<key>' })