iframes: annotations stop working when i change the iframe source

[milahu]

annotations work when an iframe is first loaded
but when i change the iframe source ...

-<iframe id="page1" enable-annotation src="OEBPS/001.html"></iframe>
+<iframe id="page1" enable-annotation src="OEBPS/002.html"></iframe>

... then annotations stop working
until i reload the hypothesis browser extension

example document
index.html is an epub reader generated by hocr-to-epub-fxl