"Wrong passphrase" when importing SSH key even if it's correct

When I try to import one of my SSH private keys (either from a file or through the system's clipboard), 1Password keeps saying that the passphrase I provide is wrong.

Providing the same passphrase, when adding another key with the same passphrase, works fine.

I'm able to add the key to the macOS default SSH agent:
bash
$ ssh-add ~/.ssh/someone@example.com
Enter passphrase for /Users/mstrzele/.ssh/someone@example.com:
Identity added: /Users/mstrzele/.ssh/someone@example.com (/Users/mstrzele/.ssh/someone@example.com)
$ ssh-add -l
4096 SHA256:VYZx99XD1lBZIDd8bgnSgMLpIQ4MQfx8wSidaL9ES1c /Users/mstrzele/.ssh/someone@example.com (RSA)


I can't add the key to the agent provided by 1Password, though:
bash
$ echo "$SSH_AUTH_SOCK"
/Users/mstrzele/Library/Group Containers/2BUA8C4S2C.com.1password/t/agent.sock
$ ssh-add ~/.ssh/someone@example.com
Enter passphrase for /Users/mstrzele/.ssh/someone@example.com:
Could not add identity "/Users/mstrzele/.ssh/someone@example.com": agent refused operation


My private key is an RSA with 4096 bytes length, as you can see in the output of ssh-add.