While my personally theory is that the Snow Queen captured her I can see one way that Anna was the one sealling Elsa away and that is if Elsa asked to her. There is a chance that the same event/s that lead to Elsa having her memory wiped by the trolls lead to her begging Anna to seal her away to protect everyone and even then it would have been a hard sell with I am presuming Kristoff acting as the middleman (assuming him dying isn’t the tragic event isn’t his death).
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“The truth isn't easily pinned to a page. In the bathtub of history the truth is harder to hold than the soap and much more difficult to find.” - Terry Pratchett