You may have Offload Unused Apps set to ON. This option will offload the apps that you seldom use and download them again if you decide to use them. This only happens when the storage space on your iPhone is low. If you have plenty of free storage space, then you can set that option OFF by opening Settings ➡️ iPhone Storage ➡️ Offload Unused Apps: Tap on it and Disable that feature.
The other option is to create more free storage space on your iPhone by deleting unwanted media files such as Music, photos and videos. These types of files take up the most space.
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Files offload
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