Adam,
I downloaded 6.1 and see the same behaviour.
A seemingly related symptom, we have also found that if there is a color set in a font tag, that for certain colors, each time the html is opened in the editor, one of the characters in the rgb string will be removed. This continues until there are only two charcters left in the rgb string at which point, the truncation will stop.
For example,
Initial text: <font color="#0000ff">Test</font>
Editor pass #1: <font color="#000ff">Test</font>
Editor pass #2: <font color="#00ff">Test</font>
Edtior pass #3: <font color="#0ff">Test</font>
Editor pass #9: <font color="#ff">Test</font>
Let me know what you think.
Thanks,
Richard