Merge should be inheriting the environment of the process that launches it, I would think. What OS are you on?
Windows, applications always get a copy of the global environment.
Linux applications inherit the environment of their parent, which would be the launcher task of your window manager, or the environment that’s current in your shell if you launch applications from the Terminal.
On MacOS applications have a constrained environment on purpose, which is inherited from the application launcher; the launcher does not share the full environment.
For this reason, on MacOS Sublime Text will launch a shell behind the scenes and gather the environment, I would assume that Merge does the same thing, though I can’t verify that at the moment.