Try this:

In the cloth room(with your cloth item selected)  select  'choreographed group' and remove all, do the same with 'constrained group' , multi-grouped vertices is a no-no.(the Choreographed group Has another function other than what you're intending.)

Now in the constrained tab select the vertices on the leading edge of your cape you wish to remain glued to your character, usually only the first or second row is plenty to get the job done.

Test your simulation:

If you see errant vertices(jaggies) select those vertices (BEFORE Clearing the simulation) and add them to the 'soft decorated' group and re-simulate.

If you're still not satisfied with the results, go to 'simulation settings' and change your 'steps per frame' to '4' and re-simulate. If that doesn't do it try 6, 8,... you get the idea...

NOTE: If this item is for personal use in your own animation, the steps outlines above are fine, If however you want to market the item, and you have errant vertices, you're selling a defective product that needs to be re-worked.

Y'all have a great day.