I am on the $30 monthly prepaid option with unlimited text/data (5gb of 4g) and 100 minutes.
The same issue occured on 4.3 back when i was using paranoid android, so i flashed to a different rom (purity rom) and it tethered just fine.
I was running that for months until the 4.4 updates came out, i flashed everything back to stock for the updates. Rooted and flashed new purity rom and a couple weeks later i tried to use tethering and here we are.
I just dont know if it is an issue now with purity rom the same that was going on previously with paranoid android, or if it is something different with 4.4 or t-mobile