Hi,
so the issue here is that the value stored in the database is a WordPress "timestamp + offset" not a true Unix timestamp as reported in the Meta Box docs and the former are being phased out from WordPress core as they're causing issues as reported here:
https://make.wordpress.org/core/2019/09/23/date-time-improvements-wp-5-3/
Also note that date_i18n() is now deprecated in favor of wp_date() as also reported in the article above.
The example code you pasted above works as long as you're not using standard PHP date functions that expect a true Unix timestamp otherwise the time zone will be messed up.
Expanding on your example here you can see that the output of wp_date() is wrong if you have non-UTC time zone in your WordPress settings:
$date_time = rwmb_meta( 'datetime' );
echo date_i18n( 'F d, Y H:i', $date_time );
echo wp_date( 'F d, Y H:i', $date_time );
Hope this make my point more clear.
