According to the USDA:
If packaging is accidentally cooked in a conventional oven, is the
food safe to eat?
Plastic packaging materials should not be used at all in conventional
ovens. They may catch on fire or melt, causing chemical migration into
foods. Sometimes these materials are inadvertently cooked with a
product. For example, giblets may be accidentally cooked inside the
turkey in their packaging or a beef roast may be cooked with the
absorbent pad from the fresh meat packaging underneath.
The giblet bag and the absorbent pad are clearly not intended to be
cooked, however if this happens and the packaging materials remain
unaltered (that is, do not melt or come apart) the cooked meat will
not pose an imminent health hazard. If the packaging materials have
melted or changed shape in some other way do not use the product.
Who are they kidding? Of course, they are altered. With deep regrets, I threw my roast away. (More info here.)