First things first, it is highly recommended to double-check if you have really cancelled your TickTick subscription successfully. This will prevent any future charges.
In general, there are a few reasons why this happens:
1. Delete the account OR uninstall the app does not mean the auto-renew has been turned off.
So here are the ways to make sure you cancel the subscription:
iOS
1. Go to your iPhone Settings > [your name] > iTunes & App Store.
2. Tap your Apple ID at the top of the screen, then tap View Apple ID (you might need to sign in).
3. Scroll and tap on Subscription.
4. Tap the subscription (TickTick) that you want to manage.
5. Tap Cancel Subscription.
For more details about cancellation, please visit the Apple Support article.
Android
1. On your Android phone or tablet, open the Play Store.
2. Check if you are signed in to the correct Google Account.
3. Tap Menu 
4. Select the subscription you want to cancel.
5. Tap Cancel subscription.
2. If you subscribed to the Premium via our website by PayPal, it is very likely that the auto-pay in Payment setting has been enabled. How to cancel
Note: After canceling your subscription, you will be able to use TickTick for the remaining days that you have already paid for. Canceling your subscription will not retroactively refund subscription payments, and previously charged subscription fees cannot be prorated based on the cancellation date. Deleting the app does not cancel your subscription.