Hi @RenataCo
This is less a Fusion question and more of a JIRA topic: We can send almost anything with Fusion, the question is what the recipient does with what se send.
Per their docs, Atlassian's textarea (ie multiline text fields with formatting) are of a special type that expects JSON
https://developer.atlassian.com/cloud/jira/platform/apis/document/structure/
It looks similar to the Draft.JS structure Workfront uses.
My suggestion, try to read a record that has HTML fields and see what the content looks like.