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Bug
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Resolution: Not a bug
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Medium
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None
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None
This means that when you update Sprint field from JIRA - Ranking is not updated - thus sorting by ranking no longer shows all items in the current sprint at the top. This affects all user's To Do filters and subscriptions (as these are sorted by Rank) meaning that they may skip something important.
You are correct, updating the Sprint field does not change an issue's rank. However, this is by design. If you would like to update rank as well as add to a Sprint, you can drag the issue in Plan Mode.