Sony gets a high bar in our listing of toxic Android vendors, as historically Sony introduced the first very effective non-standard background process optimization and opened Pandora’s box.
It is called Stamina mode, and it instantly breaks all background processes and all alarms if enabled.
Never use Stamina mode if you want your device to do something useful when not actively using it. Despite the official description, it does not affect only mobile data and WiFi, it also terminuates any background processes. Stamine mode can be find (and disabled) at either Battery section or Power management section.
Adaptive battery was reported on Android 11, but it can be present on earlier versions too.
The app you need to run in the background needs to be set as Excepted from Power-saving feature.
System settings → Apps & Notifications → Advanced → Special app access → Power saving feature
Switch Pomodoro app to Excepted
Try to make your app not battery optimized in Phone settings > Battery > Three dots in the top right corner > Battery optimisation > Apps > Pomodoro app.