Your use of the tag is correct according to the w3 docs. Make sure that it is the first element within your SVG, and it should be functional in browsers that render it as a tooltip.
As for Bing Webmaster tools, I think this is an oversight on their part. I recommend trying the same type of test using Google Webmaster Tools Fetch and Render Feature to see what Googlebot makes of it. Also running a scan from https://validator.w3.org/ should help ensure your markup is valid.
If you want to ensure tool tip functionality across browsers and avoid any possible SEO issues I recommend using a javascript implementation such as Foundation's or Bootstrap's.
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