Yes, it can self-sow and chances are much greater in a mass planting with at least two clones.....much like yours.

Thank you, gardengal48. Before removing all of them, I will take out the Issai and see if I have the same problem next year.

I have grown several plants of 'Issai' for many, many years with never a single seedling. Sounds like having two clones is definitely your problem.

I have a handful of mature Amethysts throughout the garden, and they all regularly sow. You have to get to the seedlings in the first year when they're easier to pull out. Otherwise like you said, they're tough and I usually need to use a shovel to get out even fairly small seedlings.