My daughter had a stork mark, both on back of neck (not visible once hair started to grow over it) and on her forehead, just as you've described your sons.
Same as you, she also had forceps marks and as I had never heard of stork marks at that point I didn't really know which was which, and was also too caught up in everything to care much.
The forceps marks faded and the stork mark remained, and the HV (I think) told me what it was.
I then totally stopped noticing it. I just didn't see it, despite the fact it was fairly prominent.
I can remember one friend saying "what's that mark on her head?" and I was really taken aback. Partly, because I no longer noticed it and partly, yes, I thought it was very rude.
Nobody else ever noticed it and it did fade.
I can't remember how long it took to fade, but certainly by around 1 year old, I think. It would sometimes appear when she was very cold and her face was otherwise pale. Or when she was young and screaming / red in the face.
But honestly, they do fade and it's never been an issue.
I think you have just been unlucky with rude people.