Whether leaving a mandatory workplace meeting to grab a smoke is considered rude can depend on the specific workplace culture and policies. However, it's generally not acceptable to simply leave a mandatory meeting without proper permission, especially if you have not communicated the reason for leaving.
In many workplaces, smoking is often restricted to designated areas and breaks, and stepping out during a meeting might be seen as disruptive and disrespectful to the meeting organizer and other attendees. Additionally, sneaking out without informing anyone may be perceived as unprofessional.
Being stressed during a demanding job is understandable, but it's essential to find appropriate ways to manage stress without negatively affecting your work or coworkers. If you need to take a break during a mandatory meeting, it's best to communicate your needs beforehand. Ask your supervisor or someone responsible for the meeting if you can step out briefly to take a break, and be transparent about the reason. It's important to be considerate of others' time and responsibilities.
In your case, since you snuck out without proper authorization, it seems the write-up was a consequence of your actions. To avoid similar issues in the future, try discussing your stress and smoking breaks with your supervisor or HR department. They may be able to accommodate your needs in a more suitable manner, such as allowing short breaks at appropriate times or suggesting stress management techniques that can be incorporated into your work routine.