Updates from REST aren't applied when using Custom Table

Changes aren't being stored when using custom table. If I switch to just using default storage these EXACT same REST calls work exactly as expected. This is causing all kinds of heartburn. Is this a bug or is there some other setting that needs to be set? Making changes through WP admin works fine and data is written to custom table just fine. Problem is my use case relies HEAVILY on REST. Running MB v5.6.18 and MB AIO v1.20.0

Original value:

{
    "meta_box": {
        "file_key": "123123",
        "file_name": "Basement"
    }
}

What I'm posting to CPT endpoint: (domain.local/wp-json/wp/v2/cpt_slug/428)

{
    "meta_box": {
        "file_key": "123123",
        "file_name": "Basement NEW"
    }
}

What POST response shows: (abbreviated for clarity)

{
    "meta_box": {
        "file_key": "123123",
        "file_name": "Basement NEW"
    }
}

What a GET request to same endpoint shows:

{
    "meta_box": {
        "file_key": "123123",
        "file_name": "Basement"
    }
}