My wife’s iPhone 6s has had this problem since 14.3 and is still present in 14.4. The battery percentage will fluctuate wildly going from 90 percent to almost nothing. Sometimes the phone will shut down randomly with what appears to be a drained battery, but when plugged in will immediately show a charge level over 80 percent! Looking at the graph in the battery settings, it shows the level dropping immediately to zero, and then jumping back up to some higher level. This is an extremely frustrating problem and makes the phone just about unusable if not plugged in. The battery life indicator says 86 percent, so that doesn’t seem to be the issue. This phone isn’t even two years old yet, and the problem started suddenly last week. I can’t remember for sure, but I think it was after the update to 14.3. I sure hope a fix is found soon.