Arrays not properly written to the database in 0,5% of upsert writes

When writing 242983 JSON-arrays of the form ["a","b","c", ...] (in a loop, using "upsert"), in 1295 cases (seeming non-deterministic), not an array, but an object (of the form ["0":"a","1":"b","2":"c", ...]) has been stored.

When accesing the data, the wrong datatype is returned (as stored in the database).