Steps to reproduce the issue

When creating / customising mail templates the insert tags are displayed in lowercase.
The MailTemplate::replaceTags() function expects the tags to be uppercase.

Clicking the tag link correctly inserts an uppercase tag.

But if the user manually types in the tag they are likely to type the tag in as lowercase as that is what they see in the tag list

Expected result

Actual result

System information (as much as possible)

Problem encountered in Joomla 4 and 5.
Not checked Joomla 6.

Additional comments

A short-term workaround is for the site admin to use CSS to transform the tag list text into uppercase, like this.

For the long-term be better if com_mail component displayed this as uppercase.