Notifications are removed regularly and energy saving mode activated spontaneously

I have a Samsung Galaxy A51 phone, 2 or 3 years old. Since some time, out of nothing (like just after using the phone for some time, when getting out of screen lock), I see notifications (like WhatsApp, mail, etc.) icons on the main page. However, before I have the chance to look at them/click them, the phone gets into energy (battery) savings mode spontaneously and all notifications are gone (when disabling energy savings mode manually). So, I have to find in all applications what 'news' there is. Is there a solution to get rid of this spontaneous saving mode activations? (without disabling it when I want to do it manually to save battery power) ?