I am not aware of other reasons for such behavior, apart from the ones I have described in my earlier post. Can you confirm the markup of the page is valid? It is possible that another change on the page, its user controls or master page has made it invalid and causes this problem.
The only additional idea I can suggest is setting the Pinned property to false, so the control uses position: absolute instead of position: fixed. Note that this may not fix the issue, because the quirks mode is not supported and dimension/position calculations may still fail there.
If neither helps, I advise that you share the simplest runnable version of your code that shows the problem, so it can actually be investigated.
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