Beta Affliction Warlock.
Basically it's going through an entire channel of Drain Soul before it will suggest anything else. Not suggesting a dot or Rapture when vile taint is ticking, during the channel of Drain Soul.
I think it's pretty important to suggest to use a dot/Rapture instead of going through an entire channel of Drain Soul without a single dot then to suggest using dot/rapture.

https://pastebin.com/xH6Pr74K

Also forgot to mention this is with the settings "Extend Cooldown Sweep". Which is on by default.

There's no logic in the current SimC priority that would prompt you to break a Drain Soul channel. Have you looked at your sims to see if your sim casts the whole Drain Soul?

Improvements made in 1.0.5, as discussed.