Jez2370, Just for info, we purchased this phone secondhand and we didn't realise that the screen and battery had been replaced by a previous owner.   Initially iOS 13.x seemed to work OK on it, but usability problems became progressively worse in 14.x and led to our sorry saga. As reported below, after changing the battery (n.b. using a non OEM part), all power and button issues vanished. However, problems relating to the screen remained -  insensitive and inaccurate response when typing in Messages, Mail etc.  Random things would also open up when scrolling within Safari and other apps!   There was nothing we could do in iOS to improve this.  I say "we" as several folks tried to use the phone and struggled to improve it!  I’ve finally solved all the various screen & sensitivity issues by installing an original Apple screen part .. that clearly must have been calibrated by Apple during manufacture. So in our case we found the iPhone 8 didn't play nicely with all the third party replacement screens we tried and iOS 14.x ... either that or we've been very very unlucky with them!     BTW I tried three non-OEM screens of differing brands!    Hope this helps someone else on an iPhone 8. Good luck with other models.

Just a little update that might help someone else.  After very little time using  iOS 14.2, our iPhone 8 had  very odd power issues, slowdowns and severe button lag affecting volume control and screenshots.  After installing iOS 14.3  the iPhone symptoms got worse so I decided to replace the battery.    Sadly I also had to replace the screen as a flat cable was damaged removing the battery. However having now replaced both the battery and screen all problems have disappeared and after several days not come back.  Therefore in our case it was probably a battery gradually going faulty that was leading to the lag and other issues for the phone …  However, potentially it might also have been due to some fault with the first screen.  We were ready to abandon the iPhone so pleased to have pulled the situation back!  

We have an iPhone 8 which is behaving very oddly after updating to 14.2 (updated from 14.1). After initial reboot the phone works correctly . Then after a period of sleep, sometimes within an hour and then waking, the volume buttons become unresponsive and screen-shots stop working. When it starts failing, a brief press the 'up volume' button, the on-screen volume indicator slides itself up to maximum volume. Briefly pressing the 'down button' and the indicator slides itself all the way back down and mutes! (even with finger removed!) Other hardware issues include the phone sometimes not finding a mobile service (until it is rebooted). Also the hard reset is not possible due to the (rather bonkers) volume up /down / side button combination being impacted by the aforementioned volumes control issues. Also, the phone battery charge system has gone haywire. Gaps in the charge history where it records zero charge for hours. Erratic battery charge levels being reported and sudden drops between uses .. one minute the battery showing half charge the next little left. Another problem that the phone is occasionally crashing when the charge lead is plugged in. We have reset all phone settings and removed almost every app in testing and the problem remains the same. The phone is only useful if rebooted first! Very frustrating as the phone was working fine until the last update.