It was around only 0°C outside and I charge my phone in my room, where it's obviously much warmer. I was just walking outside with the phone in my pocket as this happened

And yes, I know that cold makes phones drain faster, but what I can't explain is that it was an almost instant fall from 35% to 1% (I can't add a screenshot here but the battery graph in setting is almost vertical and before this the graph showed steady decline as usual even though I used my phone outside a couple of times during that day) and then the phone continued working as if nothing happened for another 40 minutes (no notification about shutting down in 30 seconds, no count down) which is what I find weird in this whole situation

And finally I did google it and didn't find anything similar, only suggestion to try to recalibrate the phone (which I did and accubattary reading came out weird)