It's seems, MetaBox sanitizing each attribute of field, including pattern input attribute.
Since I was trying to use complex regex with it, I got non-working result because of sanitization.
Code:
'pattern' => '(?:https?:)?(?:\/\/)?(?:[0-9A-Z-]+\.)?(?:youtu\.be\/|youtube(?:-nocookie)?\.com\S*?[^\w\s-])(?:[\w-]{11})(?=[^\w-]|$)(?![?=&+%\w.-]*(?:[\'"][^<>]*>|<\/a>))[?=&+%\w.-]*',
Rendered result (HTML):
<input pattern="(?:https?:)?(?:\/\/)?(?:[0-9A-Z-]+\.)?(?:youtu\.be\/|youtube(?:-nocookie)?\.com\S*?[^\w\s-])(?:[\w-]{11})(?=[^\w-]|$)(?![?=&+%\w.-]*(?:['"][^<>]*>|<\/a>))[?=&+%\w.-]*">
.
