Error when using vim `J`/`Join` when 2nd line starts with `>>=`

[stroiman]

Given this file contents:

  json |> Js.Json.decodeObject
  >>= doc => doc |> Json.getString("email")

If my cursor is on the first line, and I press J for Join, the result becomes

  json |> Js.Json.decodeObject = doc => doc |> Json.getString("email")

It should have been: (the >> was removed)

  json |> Js.Json.decodeObject >>= doc => doc |> Json.getString("email")

Changing the filetype to text before the join does not give any problems, which makes me believe that the problem lies with the reason plugin.