Tags in DigitalOcean used to be case-sensitive and preserved case on creation.
Recently (in the past week?) DIgitalOcean seems to be lower-casing every new tag. Is that a new, intended behavior? Is that documented anywhere?
You can easily see this behavior by going to the tags section of the Droplet and adding a tag with capital letters. It will be saved in lower case.
From what I'm seeing, lowercase is what they're using. I've not personally used uppercase with tags in the past though.
I can confirm that when trying to tag from within the control panel, anything uppercase is converted to lowercase. The same when using doctl from Terminal. Passing uppercase tags results in lowercase conversion.
There was a change a week ago, where renaming tags was deprecated.
https://developers.digitalocean.com/documentation/changelog/api-v2/deprecating-update-tag/
And reading the documentation, doesn't state that tags are only lowercase:
https://developers.digitalocean.com/documentation/v2/#tags
Maybe something changed last week, which hasn't been documented. You should create a ticket to get an official answer.