For some reason, if you only specify the _draw function but not the _update, it is not executed.
Example:

function _update() end
f=0
function _draw()
 cls()
 f+=1
 print(f,0,0)
end

Renaming _update to, say, _update1 will stop the program from drawing anything.
Most often you'll have both functions at once, but this bugs me a little.

Oh yeah, I noticed this too, but forgot to say anything here.

Same here, got the issue at first try, but thought it was normal... Thanks for sharing this.

I wonder if _draw() is an implicit call at the end of _update() in the current implementation? Complete with the conditional every-other-_update performance adjustment if the system isn't keeping up. Hence: no _update, no triggering of the _draw callback?