Iphone 7 battery issues ios 13.4, siri 100% usage

Hi, I'm a long time iPhone uses since iphone 3Gs. Now on the Iphone 11 which it great. I recently gave my son my old Iphone 7. Basically it was wiped. Added a child account. afterwards I didnt do much with it for at least a week. I updated to ios 13.4, but I didnt have a sim yet. When I got the sim then I really started looking at the phone and if it was still working fine. I have had issues with the battery life before after updating. but this time it tends to stick with ios 13.4. I have had the battery changed in 12/2008 and the battery health is now 94% (so peak performance capable).

The first day I gave my boy the phone he didnt do anything with it. I charged it in the evening till 20:29 and just left it unplugged. In the morning it was down to 23%. i then started reading different forums about issues with ios 13.4 and steps to undertake. first one was an advice that is normal with a major update. To him I can say to **** with normal 77% drain in one night (approx. 12 hours) evening is not normal.

The standard advice Apple gives is backup the phone, reset the phone. So I did. Even though the phone was practically empty, with only one non apple app on it.

Things did seem better. it went down from 77% drain to 20% drain in one night (approx. 12 hours)

but during the day. I told my son to keep an eye on the phone. He only used the phone for 10 minutes to watch an youtube video. After that he checked it once of twice to see how the battery was holding. So after (approx. 16 hours) it was down to 50%. Afterwards we left it there and starting watching a movie on the apple tv. Around bedtime I checked the phone again and it was dead. Battery empty. This is within 24 hours doing almost nothing.

What I did notice was that Siri was doing something 100% the whole time during approx. the last 50%.

I checked the settings and Siri was only on for suggestions. So i'm not sure what Siri is up to. Anyone have an idea? How can I fix this? I have turned of Suggestions and put the phone on charge again. So will have to see if this occurs again. Is this a siri, ios 13.4 bug? or part of a larger battery drain issue people are talking about on this forum? Hope someone can help. and thanks in advance.