Subject Prefix does not seem to work

I have set the email prefix to be something like ${tag:environment}

The email I have got is like the below:

The prefix does not seem to use the tag QA but it appears to be ${tag:environment}

What is that I am missing here?

This issue still appears unresolved after over a year, even though the feature is clearly documented here:

I have a subject prefix like: “[sentry-$project] $title”, yet my alert emails has this subject:
[sentry-$project] $titleNEWDEV-D - Error: The input...

I could just hardcode text into the prefix, but I want a trailing space after the “[]”, which Sentry insists on trimming.