With ReasonReact example from https://reasonml.github.io/reason-react/docs/en/intro-example the default formating in reason mode is:
[@react.component]
  let make = (~name) =>
<button> {ReasonReact.string("Hello " ++ name ++ "!")} </button>;
 
The right formatting is, obviously, the following:
[@react.component]
let make = (~name) =>
  <button> {ReasonReact.string("Hello " ++ name ++ "!")} </button>;
 
Version of reason-mode — 20190710.1037 (from MELPA).