Stuck drilling wood frame between floors
You could have hit a knot in the wood.
These screw tip bits might not be able to pull through without additional force, which you cannot apply with a flex bit. Try a different drill spot from the same entry point, 3 to 4in over sideways. If it were a nail or a plate you'd hit hard and possibly jam, but likely not heat-up.
I did a similar hole with a straight auger/spade: with the box low enough and a long enough bit you can hit the right angle. Remember, you can cut a hole the size of the ethernet cover (minus a margin). But maybe it's too late for that since you have one entry point and don't wish to patch the drywall.
Also, from one DIY to another I'd add: don't be afraid for patch drywall work. Start with a 4in hole saw, and keep the cut out. Lots of videos, and lots of help here.