I successfully extended 6 ft using this DP cable. No issues so far. Also used a long, maybe 15 ft active USB 3.0 cable I already had but I am going to try replacing it with a 6 ft cable as well. Not a big extension but I don't need much in my play space and it's cheap.
https://www.amazon.com/gp/aw/d/B01KRLQG2M?psc=1&ref=ppx_pop_mob_b_asin_title
My old set up actually put the ends of the cable closer to my PSU and monitors than my current setup does, so i dont think thats the case. Im gonna move the actual index power cable to a different outlet (a 1-to-6 plug in that i have plugged into my wall that also has one of my base stations plugged into it) and see if that changes anything.
Ive also seen things about certain DP cables not exactly working and am wondering if thats the case. although id been running it for a while with no issue.