Running the following code on the client use to work as expected on all 1.12 betas (my_custom_ui would load just fine). On 1.13.0.1 beta the code runs but the ui DOES NOT load and from the player's perspective nothing happens (note the client script runs without errors on 1.13.0.1 as I attached the VS debugger and set breakpoints).

let loadEventData = this.createEventData("minecraft:load_ui");     
loadEventData.data.path = "my_custom_ui.html";     
loadEventData.data.options.is_showing_menu = true;     
loadEventData.data.options.absorbs_input = true;        
loadEventData.data.options.render_game_behind = true;        
loadEventData.data.options.always_accepts_input = true;
this.broadcastEvent("minecraft:load_ui", loadEventData);

Also there is a discrepancy in the documentation regarding custom ui's. In the Scripting documentation (under Breaking Changes) it says to use "engine.on("facet:updated:core.scripting", ...);" while the documentation for custom ui's says to use "engine.on("facet:updated:scripting", ...);"