I have the same issue with my 6s. But I notice that then the battery low down to 1%, the battery could still be used for couple hours. I did this twice time, one time I left my phone 1% battery discharged for all night long, the next day morning it still alive, and I even used some apps, and it still functioned well. The second time was last day, I charged the battery to 100%, and I discharged it and then opened some apps and keep them working, the battery drained so fast, and it does down to 1% in 2 hours, and I continued to keep the apps work, it appeared that 1% battery can be last for 2 hours with apps functioning.
So I wondered the reason of the issue is the inaccurate calculation of the battery percentage
And I try something like this, updated the version to 13.3, used the battery to automatically shut down, then charge it to 100%, it seems helped, the calculation of the battery percentage back to normal. But I don't know if it help every iphone with the same issue, and don't know which step of those worked for my iphone, you guys can try this, it might help