Tag wiki excerpts are plain text. The description when editing tag wikis states:
The tag wiki excerpt is a brief plain text introduction to the topic that the tag represents. It is shown at the top of tag question lists, and as a tooltip wherever the tag appears. Complete this first!
This means that Markdown does not work there:
Unfortunately, for users on
/review, it is rendered as Markdown, and therefore can easily be approved although clearly attempting to use unsupported formatting (see bold text on the left):
Tag wiki excerpts in
/review should not render Markdown, as it will just lead to fancy formatting in tag wiki excerpts by users used to using Markdown, that are then approved by higher-rep users without them being able to see that excerpts aren't actually formatted.