I discussed this with mat@ on IRC who suggested I create this bug. It seems that in my case, NO_WRKSUBDIR= has no effect.
See https://gist.github.com/dlangille/509b4084aef65537453a for examples
Well, the idea is that when NO_WRKSUBDIR, each distfile should be extracted in a specific subdirectory.
Or maybe it should be an option.