Update [February 24, 2018]: Google said it found no evidence the February security patch is the reason for the reported battery issues but has acknowledged them and is still looking into the matter. The original story is as follows.
Numerous owners of the Pixel 2 and Pixel 2 XL recently took to Google's official product forums, Reddit, and various social media platforms to complain about the February security patch that they claim has negatively affected the battery life of their mobile devices. Some users are claiming their handsets have even started getting warm post-update during regular use, whereas their batteries didn't noticeably warm up prior to the new firmware being installed. A small number of people have even reported that their Pixel 2 and Pixel 2 XL units are heating up on standby, pointing to the latest security patch as the likely culprit for that phenomenon.
It's presently unclear whether all of the reported issues have the same cause as the problems vary across devices and most of them are even present while the two handsets are running in the safe mode, suggesting none of the company's apps are the culprits. Google has yet to acknowledge the issues outside of one community manager telling a single user to contact the company's customer support as they're possibly suffering from a hardware issue, though the sheer volume of the complaints that emerged online following the rollout of the February security suggests that isn't a likely scenario. A factory reset doesn't fix any of the newly reported issues, users claim, and no workarounds have yet been devised.
While monthly security patches for Android devices rarely introduce problems beyond minor bugs, the February 2018 update is much more robust than the average package Google pushes out to its products on a monthly basis; the software includes added code meant to enable the Pixel Visual Core, Google's first custom-made chip for smartphones found inside the Pixel 2 and Pixel 2 XL. The silicon designed for efficient image processing is currently available to a number of third-party apps such as Instagram, Snapchat, and WhatsApp, though it's presently unclear whether parts of its software backend are responsible for the degraded battery performance of Google's latest Android flagships. In theory, the Pixel Visual Core should conserve energy by taking the imaging workload off of the Snapdragon 835 while still allowing apps that use the Android Camera API to perform HDR+ photography.