Nope..
Please post your idw/ipt if you need help determining what the cause of that is..
The thread lines should not continue into the undercut normally..
The full thread is obviously taking it into the thread relief.
What I did is as I show below.
Windows 10 x64 -16GB Ram
Intel i7-6700 @ 3.41ghz
nVidia GTS 250 - 1 GB
Inventor Pro 2018