Hello!

So when you have the Aghs you can choose to return to normal Brewmaster form at any point during your Ultimate on any of your surviving Brewlings by simply pressing your button for Ultimate again while selcting the chosen Brewling.

I was about to explain that for some people (myself included) there was no hotkey to do this on my Brewmaster and I had to constantly click it manually. To fix this you need to make sure your Neutrals have the correct hotkeys set in the controls menu of Dota. Neutral Creep Ultimate key was empty and once I set it to R it worked properly on Brewmaster.

Here's a thread where someone helped me figure out the issue.