assign_pop() is implemented a bit oddly. In particular, the second regex starts off with

  (%%)([^`]\n?#stack:\n)

This should have just been written like the following (which in fact eq() does do, though eq() is itself missing the %%` -> %% regex):

  (%%)(\n#stack:\n)

As it is the [^`] matches the newline, which is why the \n has to be marked as optional and in practice will always be skipped.