Hmm not sure if there is an event you can subscribe to specifically for this.
To begin with, ensure the people who are not admins who can assign roles to others, ie have:

And not the Authorization > Role add/edit permissions. Ensure they don't have admin roles. I'm failing to locate the documentation on this, but feel like you can't assign roles above your own, ie if you don't have admin access you can't grant that to someone else.
You do have the audit viewer (also available through API, which you could call daily, hourly, etc) where you can keep an eye on changes to people permissions, and filter specifically by add members and on the the role ID (entity ID in screenshot) to monitor adds to just your admin role for example

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Anton Vroon
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