Escaped characters display improperly if they straddle the 'teaser_length' boundary

I have a situation where TinyMCE is escaping quotation marks, and where a " straddles the default 600 character "Length of trimmed posts" setting for displaying teasers. This leads to the quotation being displayed as &q at the end of the teaser. This bug can be circumvented by adding a teaser break, but in terms of default Drupal behavior, this is still a mishandling of a boundary case.

To replicate on a fresh 5.1 installation with the default "length of trimmed posts" setting at 600, post the following with the "Full HTML" input format:

<p><img style="width: 118px; height: 89px" src="http://tbn0.google.com/images?q=tbn:xxxxxxxxxxxxxx:http://xxxxxxxxxxxxx.org/wp-content/uploads/2006/05/xxxxxx.JPG" alt="" hspace="5" width="118" height="89" align="left" />xxxx xxxxxx xx xxx xxxx xx xxxx xxxxx xxx xxx xxxxxxxxx <a href="http://www.amazon.com/xxx-xxxxxxxxx-xxxxxxx-xxxxxxxxx-xxxxxxx/dp/xxxxxxxxxx/ref=xx_xxx_xx_x/xxx-xxxxxxx-xxxxxxx?ie=UTF8&amp;s=books&amp;qid=xxxxxxxxxx&amp;sr=8-1" target="_blank">xxx xx xxxxxxxxx</a>, xxxxx xxxxxxxxx xxx xxxx xxxxx xxxxxxxxx xxxxxxxx xxxxxx, xx xxxx xxxxx xxx xxxxxxxxx xxxxx xxx &quot;xxxxxxxxx.&quot;</p>

<p>Second paragraph.</p>