On one of my Moodle servers there are recurring errors when I insert an emoji into a text. After saving the error appears:
Error writing to database
More information about this error
The link leads to the generic page without any real help.
On another server, also running Moodle 4.5.7, the error does not occur.
Possibly you have MySQL or MariaDB but the database was not configured for full Unicode support.
Check the Environment report. Are there any warnings or errors?
If you do have a warning about Unicode support, follow the steps to migrate your database to utf8mb4 (don't forget to update config.php after running the script).