Depending upon what content is being displayed in the HTML control, I've found that tooltips are clipped by it.
Attempted to work around it by changing my layout from a box/group container to a canvas, and flipping the z-order, but that made no difference. Apparently, the HTML control is a higher level window that wants to obscure the tooltips regardless.
I didn't see any reference to this with substantial searching on the Adobe forums and the web in general. Seems like a pretty easy issue to run into. Is there any known workaround, and/or explanation of why it occurs?