Update: Apple has now acknowledged the issue on its system status page, saying:
Users are experiencing a problem with the App Store. We are investigating and will update the status as more information becomes available.
Several people are reporting problems with search in the App Store this morning, with improper results being returned. Additionally, some apps, including Spotify, Tweetbot, and Google's suite of apps, don't show up, despite still being present on the store.
According to Reddit user northernsteel, Apple has acknowledged the issue and is working on a fix:
I spoke to Apple Support, it's an ongoing issue with search they're working to resolve. Nothing has been pulled from the store.
It also seems as though the problem is impacting the Mac App Store as well, with Apple's Xcode missing from search results. We'll be sure to let you know if Apple resolves the issue. In the meantime, direct links to apps still work as intended.