Bukkit for some reason removing players from banned-players.txt?

I have a very old server, nearing 3 years old. We've had a single ban list in all this time. Recently, I became aware that people were mysteriously disappearing from banned-players.txt, I have a backup of it with over 4000 entries, but when I checked just now it had only 953 entries. I checked the logs to make sure nobody had been unbanning so many people, and there are no unaccountable unbans (less than 50 in total.)

Every player on the list is listed as "Forever". Is there some sort of expiry despite this? I mean, does Forever really just mean 1 year or something?

 

I just noticed the same thing. If I use the /seen command, it shows someone as being banned, but they are not listed in banned-players.txt. The person is listed in /banlist. /banlist shows 23 people, but banned-players.txt has 22 entries.