I’ve just edited my previous post but I answer again, you should remove the X1 certificate. The problem is that even removing the X1 certificate IIS could be serving it and it is because the certificate is held by SYSTEM user, as I said, this is the thread IIS 8.5 building incorrect chain with Lets Encrypt Authority X3 and this is the solution to remove the X1 certificate IIS 8.5 building incorrect chain with Lets Encrypt Authority X3 .
First try to remove the X1 certificate and if it still doesn’t work, try the solution in above post to remove it using the SYSTEM user.