iPhone XS keeps shutting off and wont turn on until plugged to charge

It reports 100% and battery has been changed about 3 months ago.

Hi. Can say exactly how often it turns off, got it working for 3 hours max (phone was laying and charging on desk while i set up apps and so on after migrating from android), usually its less than 30 mins, most of the time constantly (pick up the phone from charging, unplug the cable and it turns off). I cant pin the issue for any of my apps (Messenger, Instagram, Spotify), it just happens when i just carry the phone in hand/pocket. Ive made typo in original question, there is no panic logs whatsoever.

so what you wrote is crucial: after disconnecting the phone from the charger, it restarts. Such a symptom indicates a faulty "boost" circuit or other dysfunction in the charging sub-system. With no going into details, the instantaneous demand for energy exceeds the minimum levels (due to the need to switch between usb and battery (main) source). The compensator, whose task is to maintain power for a fraction of a second, necessary to switch between sources, does not work. This may be the result of using a non-original lighting cable or a remnant of incompetent assembly of the motherboard after an earlier failure. Either way, it looks like a current problem requiring microelectronics intervention.

@tojaert The thing is, i was using phone for max 30 minutes after disconnecting the charger. It doesnt turn off automatically after i unplug the cable everytime. Since it happens so randomly ive tought it may be a loose connection of some ribbon cables or faulty battery replacement. Cable seems to be original since it was unpacked but i dont know what cable previous owner was using.

so if it happens randomly, what I wrote earlier loses its validity (not applicable in this case). If this fault occurs after 30 minutes after power disconnection... (each time after 30 minutes of disconnection?) then you should consider a software problem. In this case, charge the phone to a minimum of 60%, go to service mode and restore (restore mode - this is important) system software via iTunes. Sign in to iCloud normally but don't restore apps you normally use (temporarily). Check the operation of the phone after doing so. If the phone keeps restarting after 30 minutes of use after disconnecting the power supply (if I understood you correctly - maybe write it in Polish, it will be easier :-)) you will need to consult it again. Perhaps you are reading the symptoms incorrectly and the problem is elsewhere. Look for a good repair shop in your area just in case, you may have to give it to a specialist.

but I see you've already chosen the best answer. If you have any further questions, please contact Alisha C.

It happens up to 30 mins after i disconnect the cable. Could be instantly, could be couple mins or max i guess 30 mins. Now i picked up the phone and its keeps shutting off, but yesterday i was able to use it little bit. Battery is charged 100% and that didnt seem to make much difference.

Problem nie jest stały, zdarza się, że do pół godziny jestem w stanie korzystać z telefonu normalnie, a zdarza się tak jak dzisiaj, że każda próba odłączenia go od ładowania kończy się wyłączeniem. Muszę wtedy odłożyć telefon i podłączyć go z powrotem do ładowarki żeby się włączył. Czasem jeśli od razu po wyłączeniu wcisnę przycisk on/off telefon wraca do życia ale to również zdarza się losowo. Gdy telefon działa z przyciskiem nie ma problemu, reaguje na każde kliknięcie. Wydaje mi się, że głównym problemem może być któryś ribbon cable nie wpięty do końca w złącze bądź wadliwa bateria, która powoduje jakieś mikrospięcia, które powodują uruchomienie bezpiecznika (bądź czegoś podobnego) w telefonie i odcięcie zasilania.