Hi, I see this too.
After some tinkering, I suppose it is a bug.
It affects text both in rectangle objects and in bubbles.
It happens when the background of the object is set to transparent. (Callout 1 below.)
If the rectangle or bubble has a background colour assigned, it does not happen. (Callout 2 below.)
If a callout is half on, half off another object that does have a background property, the part of the text over the object is ok, but the part of the text over the transparent part of the image is not ok. (Callout 3 below.)
Here is my image in Capture:
Capture going bold - in Capture.png
And here is the output (HTML5):
Capture going bold - output.png
Definitely odd. We should file a bug report.
You do not have the required permissions to view the files attached to this post.