Wordpress doesn't save a post revision when changing CMB2 custom fields

Still, I found it is good to update some more details on the excellent post.
Save your custom fields:

add_action( '_wp_put_post_revision', 'save_cmb2' );
function save_cmb2( $post, true ) {
// save the custom field
}

Note that the second parameter is true if you like autosave.

File: wp-includes/revision.php
275: /**
276:  * Inserts post data into the posts table as a post revision.
277:  *
278:  * @since 2.6.0
279:  * @access private
280:  *
281:  * @param int|WP_Post|array|null $post     Post ID, post object OR post array.
282:  * @param bool                   $autosave Optional. Is the revision an autosave?
283:  * @return int|WP_Error WP_Error or 0 if error, new revision ID if success.
284:  */
285: function _wp_put_post_revision( $post = null, $autosave = false ) {

You can restore a revision if you use wp_restore_post_revision function.

/**
 * Restores a post to the specified revision.
 *
 * Can restore a past revision using all fields of the post revision, or only selected fields.
 *
 * @since 2.6.0
 *
 * @param int|WP_Post $revision_id Revision ID or revision object.
 * @param array       $fields      Optional. What fields to restore from. Defaults to all.
 * @return int|false|null Null if error, false if no fields to restore, (int) post ID if success.
 */
function wp_restore_post_revision( $revision_id, $fields = null ) {

…
There is a hook defined inside that function you should use.:

File: wp-includes/revision.php
392:    /**
393:     * Fires after a post revision has been restored.
394:     *
395:     * @since 2.6.0
396:     *
397:     * @param int $post_id     Post ID.
398:     * @param int $revision_id Post revision ID.
399:     */
400:    do_action( 'wp_restore_post_revision', $post_id, $revision['ID'] );

This will lead you to the code like this:

add_action( 'wp_restore_post_revision', 'restore_cmb2' );
function restore_cmb2( $post ) {
// restore the custom field
}

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