Hi Steve,
There is a problem with the metabox builder. I am also having trouble saving on many sites.
If the "Save field value" is turned off in the Advanced section of each field, it will not save.
This field was added suddenly in a recent version, and many fields that have existed for a long time have it turned off. Please be aware of this.
* Support said that it is ON by default if you create a new file with the latest version.
We believe it is safer to set this in PHP instead of using the builder if there are many fields to support, since there could be problems related to saving with each version upgrade due to React support, etc.