Generally, you will have a tough time with multiline or pretty-printed JSON in LS, this is because it is hard to know when the last } is reached especially if there is no newline after the } character.
When you use the pattern => "^}$" (a brace at the start of a line ) with what => "previous" the text satisfying the pattern is not seen as belonging to the previous and the accumulation of JSON lines has no final }.
Having said that a pattern => "^{$" with what next relies on a timeout flush feature to flush the accumulation buffer because another \n{ is never seen. Filebeat's implementation of this timeout is better than the LS one.