Have you checked in settings > notifications > Advanced settings > floating notifications. Make sure it's on the correct setting.
Also check in phone app settings. Click 3 dots > Settings > Call display while using apps. Make sure it's on the correct one.
Might be worth clearing the phones app cache, then restart the device. If that doesn't solve could try to delete data for the phone app.
Could also try wiping the phones cache partition.
Also send feedback under support incase it's a bug.
