hybrid external mode does not work with regular mask as source

When there are hybrid external mode and regular mask together, mask works and hybrid external mode is ignored. It does not produce error. And external does not affect the output.

Bad:

$ ./JohnTheRipper/run/john --stdout --mask='[ab]' --external=hybrid_example
a
b
2p 0:00:00:00 100,00% (2019-10-10 19:26) 28.57p/s b

mask then hybrid external using temporary wordlist is ok:

$ ./JohnTheRipper/run/john --mask='[ab]' --stdout > t.wl
Press 'q' or Ctrl-C to abort, almost any other key for status
2p 0:00:00:00 100,00% (2019-10-10 19:26) 28.57p/s b

$ ./JohnTheRipper/run/john --wordlist=t.wl --stdout --external=hybrid_example
Using default input encoding: UTF-8
a_00
a__01
a___02
[...]

john is quite fresh (has opencl; openmp is disabled):

Version: 1.9.0-jumbo-1+bleeding-ea33667eb 2019-09-23 12:43:34 +0200
Build: linux-gnu 64-bit x86_64 AVX2 AC