I did not need to use a form at all, previously.
Do you think I need to put this in a form or just including the FormsModule would make this work?
I will try it.
I would consider the fact that this worked at all before to be a bug. I despise innerHTML, and wish it was deliberately dropped from Angular, but if what you are looking for is similar to what Discourse does with a preview function, I would implement it the same way. Separate the textarea where input happens, showing markup as markup, and have a separate element to display the rendered markup.