Yes, so it's due to the structure. I made several tests and I don't get any errors when updating a custom field I created in my demo, replicating the same type and details as in your account. We didn't use any of your data so it's not your custom field id:
You can use my request body below as a reference.
{
"work": {
"custom": {
"field_1698314368972": "Moon"
}
}
}
