The font size and display of text in the ATTO editor have been mysteriously overwritten in Google Chrome browser in the Boost and Snap themes, doesn't seem to affect Clean or More themes.
The editor decides to wrap sections of text with: <span style="font-size: 1rem;"> effectively locking the text to this size and not obeying other document styles.

1. Using the ATTO editor, create two paragraphs of text
2. Position the cursor at the start of the second paragraph
3. Press backspace (ie collapse the two paragraphs to one)
4. Toggle to code view. Note the addition of the <span> tag around what was the second paragraph (see sample code lower)
5. Toggle back to design view
6. Highlight all the text
7. Set style to Heading (large)
8. Note that the text inside the <span> tag does not honour the new text size

SAMPLE CODE:
<p>Paragraph 1<span style="font-size: 1rem;">Paragraph 2</span></p>

This was replicated on https://demo.moodle.net

You can see it in action here: https://www.youtube.com/watch?v=gKOB7foZA1w