For what it's worth, it sounds like wicking to me too.
The carpet will have looked wonderful when wet, when you left, but over the next few hours, as it dried out, residual soil was drawn up the pile, and that is how the landlord saw it later.
If it was a typical rental property, you might well have used plenty of water to get the dirt out; unfortunately that is the usual cause of resoiling from wicking.
The solution is to go over the carpet once more, but only lightly wetting the carpet. That way you remove the soil from the top of the carpet, but you don't draw up more from below.
Certainly you should go back.... it's only a few minutes work to re-clean and it is your reputation that you are enhancing.
In general, if you have a really dirty carpet, it can be worth telling the customer that it will need 2 cleans to get it right. The first clean is a deep clean; the second just a light "dab - and - dash"
You can reduce the risk of wicking by a good pre-spray and agitation, and waiting 15 minutes before HWE... that way the soil comes out easier and you need less HWE... but with a really dirty carpet wicking is still going to be an issue.
The landlord has no doubt had other CCs before you, and they probably had the same problem but dealt with it badly; if you deal with it well you have got yourself a customer for life.