Xiaomi

Xiaomi and their Android customization called MIUI belongs in the most troubled group on the market with respect to non-standard background process limitations and non-standard permissions.
In default settings, background processing simply does not work right and apps using them will break.

See below for how you can fix it.

Basic setting

In the recent apps tray, long-press Pomodoro app and then choose the hexagon icon

2. Make sure settings for your app look like this

3. Allow all permissions

4. Most importantly, choose "No restrictions" so the app can run in the background

Advanced for specific devices

Power management

Please enable:

Settings > Advanced Settings > Battery manager > Power plan is set to Performance
Device Settings > Advanced Settings > Battery Manager > Protected apps – Pomodoro app needs to be Protected
Device Settings > Apps > your app > Battery > Power-intensive prompt and Keep running after screen off
Settings > Additional Settings > Battery & Performance > Manage apps’ battery usage and here:
1. Switch Power Saving Modes to Off
2. Choose the next options: Saving Power in The Background > Choose Apps > select Pomodoro app > Background Settings > No restrictions

App battery saver

Security > Battery > App Battery Saver > Pomodoro app > No restriction

Autostart

Open Security app > Permissions > Auto-start