If after you have run the IdFix tool and verified that your Sync profiles have the msExchHideFromAddressLists attribute selected and still the check box in the Exchange Admin Center is not check. Pull your hair out, then follow these steps.
Here is a summary of what we are going to do. (assuming you use Office 365 already). Purchase Azure Information Protection and assign the license to any user, Connect to Exchange Online via PowerShell, Run a few commands in PowerShell, Create a Rule inside of Office 365 to encrypt messages.