Why isn't the debug logging triggered when `JULIA_DEBUG` is being set to a Main submodule?

General Usage
1 
julia> module Temp
           function f()
               @debug "in here"
               return
           end
       end
Main.Temp

julia> ENV["JULIA_DEBUG"]=Main.Temp
Main.Temp

julia> Temp.f()

julia> ENV["JULIA_DEBUG"]=Main
Main

julia> Temp.f()
┌ Debug: in here
└ @ Main.Temp REPL[1]:3

Why was the debug message not printed when JULIA_DEBUG was set to Main.Temp?