No. It is not new. VPN use sometimes interferes or prevents the user from posting comments on Youtube.
You should post your question on the Youtube community support forum to see other similar reports. Your issue can arise from multiple things, but in the context of your post, it appears that you can add comments while not using the VPN.
Welcome to the YouTube Help Community
However your best bet is to submit a support ticket directly to Youtube for your paid premium account:
Get support for YouTube Paid Products
Opening a case there they will direct you to the required department.