We've never seen this before with any of the handmade wheels leaving our shop, until one instance just a few days ago, and now yours! That's why we're thinking it could be something to do with the nipples we used, possibly a production error.
I’ve heard of a few similar issues where a modern, wide, lightweight tubeless rim has been used with a tyre at really high pressures, which effectively reduced the overall ERD of the rim and decreases spoke tension markedly. This could then lead to spokes slackening off further (as undertensioned nipples tend to do), and the above issue.
Might not necessarily be the case here, but definetly worth bearing in mind with other cargo wheel builds, that tend to use wide rims and see higher pressures than these rims are designed for.