Nokia Mobile has started the rollout of March 2019 security patches to a bunch of phones. In this wave, the Nokia 5.1 Plus, Nokia 7 (China), and Nokia 8.1 are getting the update. The update for Nokia 5.1 Plus seems to be a security patch alone as it weighs 84.6 MB. Similarly, the Nokia 7 is also getting a 106MB update with security patch alone.
However, the Nokia 8.1 is getting a slightly larger update of 126.2MB. Since the size is slightly larger than usual, this does suggest some bug fixes are included with the update.
If you own any of the three phones, then head over to Settings > System > System update to download and install the update onto your Nokia phone. Alternatively, you can wait for some time, and a notification will alert you regarding an available update.