This is because usually with major OS updates, indexes need to be rebuilt and this can take time, which can also result in battery drain. This might resolve itself eventually, but in the case of the recent iOS 14 update, it appears that Apple is acknowledging that there could be battery drain as a result of the update.
So much so that the company has published a support document on its website in which they help users troubleshoot some of the various issues related to the update. Unfortunately, for now it seems that the only solution to the battery drain problem is basically to wipe your iPhone. This is kind of extreme and we don’t think that it should be done lightly.
We suppose it depends on how badly this battery drain is affecting your usage. If you don’t find that it impacts your day-to-day usage too badly, then maybe you might just want to hold off and wait for a fix. However, if the drain is affecting how you use your phone, then maybe you might want to consider doing a wipe and see if it helps.