It doesn’t breach E2EE; it gives your employer control over the device. Once messages are decrypted on the phone so you can read them, anything your employer deploys via MDM (screen capture, keylogging, backup/forensics tools, admin unlock, etc.) can potentially copy them.
On a company-owned, fully managed device, you should treat MDM as roughly equivalent to handing your boss an unlocked device: anything you can see on-screen could be captured or exfiltrated by tooling they deploy.
On a company-owned, fully managed device, you should treat MDM as roughly equivalent to handing your boss an unlocked device: anything you can see on-screen could be captured or exfiltrated by tooling they deploy.