Destination tag isn't always required and that's why Bitsler performed your withdrawal without destination tag.

Destination tag is usually required when you send XRP to exchanges or custodial wallets (or any other third party)
Exchanges and custodial wallets usually give all users a same address and give a unique destination tag to each of them to know who has deposited.
Since Bitsler cannot know whether you are sending XRP to your own wallet, an exchange or a custodial wallet, they cannot know if a destination tag is required.