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That could be a problem, yes. static.domain.com is another domain that www.domain.com and therefore the sitemap should be in www.domain.com for URLs on www.domain.com. I've seen image sitemaps in www.domain.com where all the images are on a subdomain, like static.domain.com and that wasn't an issue, but personally I'd suggest you put the sitemap in the www.domain.com.

Thanks Sorina!