hybrid mask with truncation by --stdout=N strange behaviour

Reposting with minor modifications from comment to #3731.

Same lengths of passwords, same options, but different letters cause different results.

$ printf 'abcdef\nabcde\nabcd\n' > wl1

$ printf 'aaaaaa\nbbbbb\ncccc\n' > wl2

$ cat wl1
abcdef
abcde
abcd

$ cat wl2
aaaaaa
bbbbb
cccc

$ john --wordlist=wl1 --stdout=9 --mask='?w?W'
Using default input encoding: UTF-8
abcdABCD
1p 0:00:00:00 100,00% (2019-04-05 23:11) 12.50p/s abcdABCD

$ john --wordlist=wl2 --stdout=9 --mask='?w?W'
Using default input encoding: UTF-8
aaaaAAAA
bbbbBBBB
ccccCCCC
3p 0:00:00:00 100,00% (2019-04-05 23:11) 27.27p/s ccccCCCC

$ john --list=build-info
Version: 1.8.0.16-jumbo-1-bleeding-7187208 2019-04-05 21:04:49 +0200