Exactly the same problem on an iPhone 6s Plus and iPhone 7 Plus. There is definitely an issue with the most recent OS release in that it absolutely drains the battery. I experimented with wiping the phone completely and doing a net new install with no new apps and the problem remained (battery drains very quickly). Also, now the "Important Battery Message" is saying your battery's health is significantly degraded yet I replaced the battery on the 6s Plus just 1.5 years back. Another Apple OS14.3 "benefit" is that the phone will periodically automatically "reboot" (black screen with spinning dial and then Apple logo) when just using the phone as I have used it for the past 4+ years. Clearly a move to obsolete older phones and get people to upgrade or just poor quality control (not sure which is actually worse). Bad form Apple - shame on you!