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I am trying to reproduce your issue.

Created a table on MySQL 8.0 with this definition:

CREATE TABLE `curdate_test` (
  `id` int unsigned NOT NULL AUTO_INCREMENT,
  `title` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_general_ci NOT NULL DEFAULT 'Untitled',
  `when` date DEFAULT (CURDATE()),
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_general_ci;

When I do a SHOW CREATE TABLE `curdate_test` I do get the default value expression in parentheses – in fact, what I pasted above is exactly that, as I forgot to write down my create command which simply lacked the charset and collation compared to what MySQL spat out.

When I take a backup of this site, the SQL dumped is exactly as I have it above. This makes sense, since we just run a SHOW CREATE TABLE and save the result in the SQL dump file. We only construct ourselves the INSERT commands for the table data.

Restoring this backup on MariaDB also works.

I am not sure what is happening in your systems, but it's not something I can reproduce following your instructions, I'm afraid.

The MySQL version is not as important, since you told me that the column definition is DEFAULT (CURDATE()), with parentheses. As I told you, we just store the CREATE TABLE command as the SHOW CREATE TABLE command spat it out, changing the table name (replace the prefix with #__) and foreign key constraints (replace the prefix with #__). We don't change the DEFAULT value. We have no reason to.

So, was your table defined the way you said it was defined, or not? If you are adamant it was defined with a default value expression in parentheses then what you described is impossible to reproduce.