Have you tried force stopping and clearing cache for the keyboard app, then rebooting the device? If the issue persists, please send us an error report. Thank you.
One UI Beta Team
I guess Android 11 deprecated custom "toast" notifications, which some apps still generate, but because Android 11 won't let them, it comes up as "null" toast. there may be an app settings to disable this
I tried clearing cache, reinstall, force stop, samsung keyboard and gboard. it happens in any app when the keyboard pops up. is there a better way to dump a log for you?