this is a statistical error or due to some other reason.
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
However, within the spending section, my API usage has not increased at all. I am concerned that one day, the entire Ultra quota might be suddenly deducted, preventing me from using the advanced models.
That’s correct and expected. $0 spending means your Ultra subscription is covering all your Opus usage. There’s no hidden accumulation or delayed billing - usage is tracked in real-time. If you ever exceed your included allocation, on-demand charges would appear on the spending page immediately, not retroactively.
In short: $0 spending = your plan is covering everything. No sudden deduction will happen.