According to the tooltip on the Dotted mob, the haste does not last the whole duration just when Dark Soul ends.
For example, when I cast Agony it ticks every 1.71 sec and with Dark Soul it ticks every 1.31 sec. I recasted agony right before Dark Soul ended under the impression that it would tick for 1.31 sec the whole duration but it did not, it reverted back to 1.71 sec once Dark Soul ended.
Is this just a tooltip error or does Haste update dynamically? I was under the impression it did not.