Faulty 'text_list' update

Hello!

I created a table like this (relevant columns shown only):

MB_Custom_Table_API::create( "{$wpdb->prefix}_characters", array(
        'height_known'          => 'BOOLEAN',
        'height_imperial'       => 'TEXT',
        'height_metric'         => 'DECIMAL(5, 2) UNSIGNED NOT NULL',
    ), array() );

And then I created a metabox like this (relevant custom fields only):

$meta_boxes[] = array(
        'title'         => esc_html__( 'Characters Extra Info', 'my_text_domain' ),
        'id'            => 'characters-extra-info',
        'post_types'    => array( CHARACTER_CPT_SLUG ),
        'context'       => 'normal',
        'priority'      => 'high',
        'autosave'      => true,
        'storage_type'  => 'custom_table',
        'table'         => "{$wpdb->prefix}_characters",
        'fields'        => [
            [
                'id'        => 'height_known',
                'type'      => 'switch',
                'name'      => esc_html__( 'Height known?', 'my_text_domain' ),
                'std'       => 0,
                'style'     => 'square',
                'on_label'  => 'Yes',
                'off_label' => 'No',
                'columns'   => 4,
                'tab'       => 'biological-tab',
            ],
            [
                'id'      => 'height_imperial',
                'type'    => 'text_list',
                'name'    => esc_html__( 'Imperial height', 'my_text_domain' ),
                'options' => [
                    'ft' => '',
                    'in' => '',
                ],
                'columns' => 4,
                'tab'     => 'biological-tab',
                'visible' => [
                    'when'     => [['height_known', '=', 1]],
                    'relation' => 'and',
                ],
            ],
            [
                'id'          => 'height_metric',
                'type'        => 'text',
                'name'        => esc_html__( 'Metric height', 'my_text_domain' ),
                'placeholder' => esc_html__( 'cm', 'my_text_domain' ),
                'columns'     => 4,
                'readonly'    => 1,
                'tab'         => 'biological-tab',
                'visible'     => [
                    'when'     => [['height_known', '=', 1]],
                    'relation' => 'and',
                ],
            ],
        ],
        'tab_style'     => 'default',
        'tab_wrapper'   => true,
        'tabs'          => array(
            'biological-tab'  => [
                'label' => 'Biological',
                'icon'  => 'dashicons-admin-users',
            ],
        ),

Here's the problem: the first time I set the 'height_imperial' field, everything is fine. The database column is NULL and it all works out nicely. For values "5 feet 7 inches", the database column will be:

a:2:{i:0;s:1:"5";i:1;s:1:"7";}

The next time I update the post, regardless of whether I change the values or not, the database changes to:

a:3:{i:0;s:0:"";i:1;s:1:"5";i:2;s:1:"7";}

That extra "0" at the beginning of the array causes the values to shift and the field is not displayed properly anymore, and it's saved wrongly from now on.

I was able to confirm with the filter 'rmwb_height_imperial_value' that the values passed on is correct, namely array(5,7) AND that it all works out perfectly if I disable the custom table storage.