I don't recall seeing this reported. Apparently ADE doesn't like putting floats in the first line (even when there is only one line), unless the float is the first element in the block.
Code:
p span { float: right }
<p><span>TEST</span>A test paragraph with a float
as the first element. It is correctly floated, and appears
in the first line.</p>
<p>A<span>TEST</span> test paragraph with a float
not as the first element. It is correctly floated, but
moved to the second line.</p>