Snap Inc. is one of the notoriously large companies that solidly rely upon doing server-side, or in other words, A/B testing for their latest and greatest software innovations. Snapchat's most recent experiment consists of a mild revamp to the bottom section of the UI when their famous and fancy lens sticker set is in use.
Discovered by our editor-in-chief, Alexey Shabanov, with the newest 10.80.1.0 beta build of the Snapchat app installed on his Samsung phone, there are indeed four bottom tab buttons when lenses are active, instead of the previous "Chat" and "Discover." Here's more info posted from our official Twitter account:
Furthermore, there are multiple changes to the discover section for third-party lenses. Remember that you can add as many lenses as you want, all made by Snapchat Creators. More on that on their Lens Studio site.
To reiterate, these UI changes are in testing and there is no 100% guarantee that you'll get to try them out by just becoming a tester. You can grab the newest beta from APK Mirror and you'll also need to have APKMirror Installer on your device to be able to install the downloaded .apkm file.