Should I be concerned if my boyfriend followed his ex on instagram again?

I’ve been with my boyfriend for almost five months now. He’s talked to me about how his last ex broke his heart when she cheated on him after 1 year of being together when he was in the military. They both unfollowed each other shortly after they broke up. Today, I saw a notification on his phone that his ex girlfriend accepted his follow request on instagram and I saw that she followed him back. Should I be worried that he still has feelings for her/might cheat on me with her?
Should I be concerned if my boyfriend followed his ex on instagram again?
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