Tapered Threads - (again)

Apologies, but there is something escaping me in creating a tapered thread hole inside a solid.

I have created many parallel threads without issue and will demonstrate that now.

I planned to print a test block in 3D, with a number of holes to test for fit.

The left most hole has a cutter element shown highlighted in brown.

Selecting a Pitch of 1.41mm - the same as the cutter, with revolve helix and cut selected with a depth of -20, a draft outline of the thread is shown 

Selecting Full pull gives you a nice thread as shown below.

Selecting the same details and adding a taper of 1.8 degrees, all looks good before you set the cut in motion.

If you watch the video below, you will see that the threads starts to create with some sort of internal structure.

The video takes a few seconds to run, if you continue watching, the thread and the cutter tool eventually disappears without creating a thread.

I guess that I am making a schoolboy error, but am struggling to see what it is.

Hopefully someone can point out the error I am making?

Many thanks

Hugh