R, 50 bytes

split(r<-readLines(),substr(r,1,regexpr("\\]",r)))

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Outputs as a list with each element named with the [tag]. Each list element maintains order within its tag.

R, 50 46 bytes

function(r)split(r,substr(r,1,regexpr("]",r)))

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Outputs as a list with each element named with the [tag]. Each list element maintains order within its tag. Returns an empty named list named list() for empty input.

-2 bytes each thanks to Robin Ryder and Dominic van Essen!