[{"key":1,"array1":[1,2,3],"array2":["a","b","c"]}]

db.collection.update({},[{$addFields:{matchingIndex:{"$indexOfArray":["$array2","b"]}}},{"$addFields":{"array1":{"$cond":{"if":{"$lt":["$matchingIndex",0]},"then":"$array1","else":{"$concatArrays":[{$slice:["$array1","$matchingIndex"]},{$slice:["$array1",{$add:[1,"$matchingIndex"]},{$size:"$array1"}]}]}}},"array2":{"$cond":{"if":{"$lt":["$matchingIndex",0]},"then":"$array2","else":{"$concatArrays":[{$slice:["$array2","$matchingIndex"]},{$slice:["$array2",{$add:[1,"$matchingIndex"]},{$size:"$array2"}]}]}}}}},{"$unset":"matchingIndex"}])