Was there an early preview of Kagamigami or a similar series in Shonen Jump?
I suspect you may have read Shikigami Twilight Days (式神トワイライトデイズ), published in 2014 vol. 28, which is the one-shot predecessor to Kagamigami.



In case you're interested, I found this by using this nifty Shounen Jump database and searching for the author (Iwashiro Toshiaki / 岩代俊明) in the oneshots (読み切り) section at the bottom.

Kagamigami's dic.pixiv page verifies that Shikigami Twilight Days was the one-shot version of Kagamigami.