Hi YamHayman,
We understand your keyboard disappears from the display when you need to use it with WhatsApp. Does this happen with any other apps? Try to close the app, update the apps manually, then restart your device and test it again.
Close an app on your iPhone or iPod touch
How to manually update apps on your Apple device
Restart your iPhone
If the issue remains, make any necessary adjustments that will help allow the keyboard to remain on screen.
Adjust the onscreen and external keyboard settings on iPhone
Also, consider reaching out to the apps developer to report the issue if it continues, but is isolated to WhatsApp.
How to contact an app developer
Thanks for posting in the Apple Support Communities forum, enjoy your week!