Mind bender exists in the 15s, but the inescapable torment can extend it. However, during that extended time (e.g. total time —20s, extended time—5s), Penance won’t transfer to shadow spell as it should.
So apparently selecting the Mindbender talent reduces Shadow Covenant’s buff duration down to 12 seconds, even though Voidwrath’s duration is 15 sec. Without Mindbender selected, Shadow Covenant’s duration is 15 sec and still gains the extra time from Inescapable Torment as it should, which would match the length of time the Voidwrath or Shadowfiend are active for.
So you’re right, it’s just that Shadow Covenant’s duration isn’t matching Voidwrath’s duration when Mindbender is selected.