Your ex boyfriend of over a year may be reaching out to your new boyfriend to tell him bad stuff about you because he is doesn’t want you to be happy.
This is especially true if he isn’t happy himself.
He may be upset that he is still unhappy and possibly hasn’t found anyone better to replace you as a girlfriend, while you are now with this new boyfriend and possibly happier with this new boyfriend than you were with him.
There is a sense of jealousy and resentment that comes with that.
If your ex boyfriend was happy in his life and with a new and better girlfriend, he wouldn’t have felt the need to send the message, “be careful, she is dumb,” to your new boyfriend.
He is just not at a happy place in his life right now.
If he had a relatively bad breakup with you over a year ago, the last person that he would want to be happy at this time is you.