I have found in my own projects and sometimes in other people's projects when I open them preload doesn't work.
I get errors like this:
E 0:00:00.718 load: Failed loading resource: res://Weapons/Fists.tscn. Make sure resources have been imported by opening the project in the editor at least once.
<C++ Error> Condition "found" is true. Returned: RES()
<C++ Source> core/io/resource_loader.cpp:270 @ load()
Even when I can find the scene at the path.