This is expected behavior. On the Ultra plan, your Opus usage is part of your included plan allocation. The “API usage” section on the dashboard tracks on-demand spending (usage beyond what your plan covers), which is why it shows no increase - your Opus usage is fully covered by your subscription.
To see your actual usage, check your dashboard — it breaks down your included usage vs. any on-demand spending. You should see your Opus requests reflected there under included usage.
Another user asked the same question here: Is Usage Accurate?
For more details: Usage and limits