Short answer
Your eSIM works while both conditions are true:
You still have available data.
The data validity period has not expired.
Subscription validity and data validity are not always the same.
In many cases, the data validity period starts when the eSIM first connects to a supported local network in the destination, unless the app shows another rule.
Example
If you have a 15-day subscription and the eSIM first connects on day 10, the data validity period may start from that first successful network connection.
To avoid unwanted renewals, turn off auto-renew in Settings → Manage Subscription.