Hi UPDATEISSUESssss,
Thanks for contacting Apple Support Communities!
We understand that you're unable to remove the keyboard from Messenger when you're on a Messenger call.
Since Messenger isn't an Apple developed app, we'd suggest that you contact the app developer to report this behavior. If you need help to locate their contact information, you can follow the steps outlined here to do so: How to contact an app developer - Apple Support
"Contact the developer of an iOS, iPadOS, or watchOS app
- Open the App Store on your iPhone or iPad.
- Search for the app, then tap it. If you can't find it, tap the account button
or your photo at the top of the screen, tap Purchased, tap "Not on this [device]," then tap the app. - The app's product page appears. Scroll to the Ratings & Reviews section, then tap App Support. If you don't see App Support, make sure that you're signed in with the correct Apple ID."
Take care!