hashcat skipping just the last few candidates in a combo attack....how come?

so I'm attempting a combo attack with hashcat, trying to generate candidates like 1stword2ndword123. here is my command line:

hashcat -m 2500 -a 1 -r bf.rule /home/me/hccap/some.hccap /home/me/wordlists/allwords.txt

bf.rule created with maskprocessor to append 3 digits: ./mp.bin -o bf.rule '$?d?d?d'

the last four words in my test wordlist are:
abuse
abused
acacia
accept

--stdout shows this works pretty well, but it's skipping the last few candidates, and I can't figure out why:

abandonabused996
abandonabused997
abandonabused998
abandonabuse999
abandonacacia996
abandonacacia997
abandonabused998
abandonabuse999
abandonaccept996
abandonacacia997
abandonabused998
abandonabuse999

it skipped
abandonabused999
abandonacacia998,999
abandonacept997,998,999

and duplicated some others, like abandonabuse999

what, if anything, am I doing wrong here that would produce this behavior.....I am truly stumped, and any help MOST appreciated!

db60