Hi, since the version 5.1 of Meta Box, we have added a sanitization for all inputs to improve the security. Textarea by default will have the wp_kses_post
sanitize callback, which will strips all iframe, script and style.
To bypass the sanitization to make it accept iframe, please add this setting to the field:
'sanitize_callback' => 'none'
See more in this docs: https://docs.metabox.io/sanitization/#bypass-the-sanitization