Hello mathskt,
Ensuring Focus remains on is important and you've come to a great place for help. With Focus, you can enable Focus Status in Settings > Focus > Focus Status. This will allow your selected app, such as Messages to notify your contacts that you have your notifications silenced. These steps can also be found listed below and in the following article.
"To turn Focus status on:
- Go to Settings > Focus.
- Tap Focus Status.
- Turn on Share Focus Status.
- Select which Focus options can share that you have notifications silenced."
Use Focus on your iPhone or iPad
If you'd like to provide feedback about the different Focus settings, you can do so via the Product Feedback website. Apple appreciates all feedback regarding their products, services, and features.
As for the Focus turning off unexpected, begin with restarting your iPhone. While restarting is a simple step, it can help with resolving many app related issues.
Restart your iPhone
Let us know how that goes!