iPhone SE: Quick battery drain when not using

Dear community and iFixit-team,

I need your help.

I have an Iphone SE with IOS 12.3.1 which discharges extremely fast. This happened from one day to the other and was not correlated with dropping, updating the phone or any other obvious happening.

Comparing with my friend’s Iphone SE (same age, same usage, same remaining battery capacity), my phone loses 60% battery in flight mode in 6 hours as compared to 3%.

This was, what I already tried:

• Charged phone to 100% and switch off over night. After turning on, the battery was still charged to (close to) 100%. This should exclude short circuits.
• Fully reset phone and tested discharging with nothing but the bare default apps but battery still drained. This should exclude any battery-draining apps or settings.
• Replaced battery using the iFixit replacement kit (thanks for the nice guide!!!) even though the old battery still had 86% remaining capacity. Same draining rate as before. This should exclude a bad battery.

Now, I don’t know what to check next.

Any help , direction or hint is very appreciated!

Thanks in advance for your support.

Thomas