FYI
After Sept. 18, you’ll no longer be able to send or receive direct messages on YouTube as we focus our efforts on public convo tools (comments, posts, etc).
You can still share what you’re watching w/ others via the Share icon below videos.
More→ https://yt.be/help/9Hns
Strange, we'd like to check this further. Mind sharing your channel URL, a some screenshots of where you see the notifications? As a general info, there are a few requirements for monetization & you can learn more about it here: https://yt.be/help/ECX1