My panel version is 6.8.27 .But I had the same issue with previous versions also. My database tables are in utf8mb4. I checked the database back up but they are in utf8.
By default the database is backing up in utf8 only. Currently I am running below customized cron mysqldump -u databasename -p --default-character-set=utf8mb4 to back up the database to forec utf8mb4 character set and when I restore from that back up, everything is fine. But there is no way to force utf8mb4 while using one click migration tool.There should be an option to select the default character set with back up, otherwise evryone with utf8mb4 character set will be affected. if your database is utf8 , then the back up will be normal. You can test back up with a utf8mb4 database and verify