Error whitelisting subnet to Key Vault

Question

I'm having trouble whitelisting a new subnet to my Key Vault. I've previously added other subnets from other vnets without any issue. For some reason, for this vnet and subnet I'm getting an error.

I'm in the Networking settings of my Key Vault (via 'Manage' button on Networking page). Note that I have previously added a few subnets successfully via the exact same procedure:

I select the virtual network I want to add:

I click "Add" and it appears in the list:

When I click "Save" I get this error:

I don't understand how come I don't have sufficient permissions, as I was able to add the other subnets AND I have the Owner role in both associated Subscriptions (the one where the Key Vault is, and the one where the Virtual Network is). Checking with "View my access" in the IAM tab for both resources:

I also compared my roles/permissions to the other vnets that I WAS able to successfully add to the Key Vault, and there's no difference at all, so I don't understand why it's failing: