All tags in Joplin are lowercase. I guess the plugin lets you create a tag with capital letters but when Joplin saves it the tag is converted to lower case.
So a potential improvment could be to inform / alert the users when creating tags. So all users can work smoothly with this Joplin plugin, even those who are not familiar with programming.
Or allowing tags in uppercase within the Joplin main code, might be more relevant.
This has been discussed several times here and in summary - not going to happen.