HI Jessica,
this only happens when you
- select any of the formatting options from the extra keyboard buttons
- select "Format Document" from the action menu
In both cases you tell Notebooks to handle the current note as a Markdown formatted note, and whenever you hide the keyboard, Notebooks will convert the note to HTML.
To revert a formatted document to plain text, just select "Copy as Plain Text" from the action menu.
To avoid the conversion you might consider hiding the extra buttons (from Notebooks' settings), because you cannot accidentally touch any of the formatting buttons.
Alternatively, you might also take a look at the Markdown chapter in our handbook to find out which formatting options you have, maybe you find them useful.
Best,
Alfons