Last modified: 2014-03-04 12:01:09 UTC
If you reblock a user, the autoblock of the IP address (as listed in Special:BlockList) will be removed. You will then need to unblock and block the user again (with the intended settings). This can happen when a user abuses their talk page for instance and you need to select a different block option.
It is a side-effect of this bug: https://bugzilla.wikimedia.org/show_bug.cgi?id=5445
You have provide no version, but this should already be fixed on master with Gerrit change #66366, because it replaced the delete/insert with a update
Oh, I guess I should have reported it sooner. Sorry all.