we have just noticed that characters like ✓ are being saved in the title field but not in plain text fields regardless of the database type (PostgreSQL / MySQL) or field type in the database (text, varchar etc.).
Thanks for your help, Lars
The Plain Text field only supports emojis that the LitEmoji library supports:
https://github.com/craftcms/cms/blob/develop/src/fields/PlainText.php#L252
Unfortunately, ✓ currently isn't one of them.