Hello Wk7151,
That's definitely not expected behavior so we'd like to help. While there is a Split Keyboard option on an iPad (If your iPad keyboard is small or split in half), that isn't the case on an iPhone.
As with all unexpected iPhone behavior, try taking the following steps which may help with this: restart your iPhone, make sure it is up to date, close the affected app (App Store and any others you experience this with), then reopen it and make sure all apps are updated.
Restart your iPhone
"Press and hold either volume button and the side button until the power-off slider appears.
Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo."
Update your iPhone, iPad, or iPod touch
"Back up your device using iCloud or your computer.
Plug your device into power and connect to the internet with Wi-Fi.
Go to Settings > General, then tap Software Update.
If you see more than one software update option available, choose the one that you want to install.
Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now."
Close an app on your iPhone or iPod touch
"From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
Swipe right or left to find the app that you want to close.
Swipe up on the app's preview to close the app."
How to manually update apps on your Apple device
"Open the App Store.
Tap your profile icon at the top of the screen.
Scroll to see pending updates and release notes. Tap Update next to an app to update only that app, or tap Update All."
Let us know if we can help with anything else at all or if this doesn't settle your issue so we can look into this further with you. Thanks and have a fabulous day!