The Send Custom Notification action's body only accepts plain text, and will display any markup as actual text like you see in your results.
I suspect your {!Body} resource in the Flow is a text template, correct? If so, you can open that text template resources for editing, and toggle the drop down that says "Rich Text" and change it to plain text. This way, no markup or special characters will get added to the text you place in the text template content (other than merging in the values of any resource you place there at runtime)
Hot tip:
Before you toggle it, while it's still Rich Text, I recommend highlighting the content you have, use Ctrl+X to cut it (or Ctrl+C, then delete), and THEN toggle the empty content to plain text and then use Ctrl+P to paste your content back in. There's some funkiness otherwise (you'll get all that markup suddenly displaying as plain text like you see in your notification)
Once your text template is switched to plain text, you should be good-to-go!