Currently, there is a bug within the Pixel Launcher that is causing some apps to disappear at random, for some users.
This bizarre bug came to Pixel devices with the December security update. It's likely that Google will fix this with the January update that is due to come out next Monday. So keep your fingers crossed.
This bug doesn't appear to be affecting everyone though, for instance, I haven't experienced this on my Pixel 4 XL running the December update. But others have. Which makes this bug even more bizarre.
Affecting Pixel 2, Pixel 3a and Pixel 4 smartphones
According to the reports that we have seen, this appears to only be affecting the Pixel 2, Pixel 3a and Pixel 4 line of smartphones from Google. So the Pixel and Pixel 3 lines seem to be fine.
When the icon disappears, the name of the app is still there, but the icon is invisible.
However, once you tap on the invisible icon, it does appear again. Which is really odd.
Users over on Reddit have reported that this also happens with folders. So your folders could also disappear thanks to this bug.
It will most likely be fixed with the January 2020 update
This bug will most likely be fixed with the January 2020 update that should start rolling out next week. Though, given the recent history with Pixel security updates, that might not be the case. Seeing as some Pixel 3 and Pixel 4 units are still on the November and October updates, respectively.
Google has at least confirmed that the Pixel 3 and Pixel 3 XL will get a combined security update in January that brings December and January updates to the phone. But we're still unsure about the Pixel 4. Seeing as most units have not received the December update, unless they sideloaded it (like myself).
It's a bit odd that this bug is only affecting some users, and not all. It's even more odd that it's happening in the first place. Though, it could be much worse. At least this bug doesn't keep you from being able to use your smartphone. Hopefully Google is able to push out an update to the Pixel Launcher to fix this bug that's making apps disappear relatively soon though. While it's not a show-stopping bug, it is a pretty annoying one. Imagine unlocking your phone and wanting to tap on Twitter to check out what's happening, but you don't see the icon? Pretty annoying. You can still tap on it though.