How HTML Tags Can Get Into a Text Template Body Field
Text template are always stored as text in flow metadata but that plain text may have html tags in it. These tags get added in the following ways:
1) if you are viewing the text template in Rich Text mode, any formatting you apply using the toolbar will cause html tags to be added to the underlying text.

In some circumstance HTML tags are automatically added even when you don’t obviously apply a format. Simply clicking on the body of the text while in Rich Text mode will automatically cause the underlying editor to add some <p> tags if they are not already there. And if you hit the return key, the editor will automatically add </br> ad <p> tags.

Preventing HTML Tags From Being Saved in a Text Template Body Field
The only way to be sure that your flow metadata will not contain tags that would become visible if the text template was passed to a plain text consumer like the existing Send Email or Post to Chatter actions is to either:
1) Set the text template to Plain Text before entering any text or
2) Change the text template to Plain Text and hand-remove any resulting tags that become visible.

Removing Unwanted HTML Tags
If you have accidentally created a Text Template set to Rich Text but actually want Plain Text, follow these steps to change it:

1) Select your current Body text and Copy it. Then delete it from the Body field
2) Use the Text Type menu (see image below) to specify Plain Text
3) Paste the text back from the Clipboard into the Body field