What's happening is the tspline is going into box mode, because your creating an invalid tspline body by deleting that face. You will have to figure out what is incorrect about the tspline in order to get it to go smooth again. I can see other issues with your model in the pic, but one thing at a time.
If your new to tsplines, I would suggest starting with much simpler geometry. Tspline modeling seems really simple, but there is way more to it than most people realize at first. Getting a good shape requires a bit of for thought and knowledge. It's very similar to Sub-D modeling. There is a lot of info out there on good sub-D modeling that applies to T-splines.
Here is some vocabulary related to tsplines. Understanding these terms is essential to creating a good tspline model-
-Star Point
-Topology
-Loop
-Quad face
-Ngon
-non-manifold
post your model for more help
