Most DBS setups right now don't do any sensing. They just send a regular pulse that is manually adjusted. It doesn't really matter that much that there is a small amount of scar tissue around the electrode, the scar tissue is also a conductor. And, that area of the brain is likely pretty damaged anyway.

Sensing DBS with closed-loop feedback is very new. But there you're looking to sense at a totally different resolution. With something like a Utah array, they're looking to record extremely accurately from a very small number of neurons, like individual neurons. With DBS sensing they're looking to get the average response from somewhere between 1k-100k neurons. The scar tissue in the first case can overwhelm your signal, in the second case, it's a small fraction of the area you're recording from.