Never had any issue with it before.

If a hacker would like to hack into your account.
1 - He needs to know your email and password to your login.
2 - If he access from another browser which probably reach the limit of simultaneous login (Shopee control that), the browser will send OTP to your registered number.
3 - If he got the OTP, only he can access into your Shopee account
4 - Then the hacker needs to have the 6 digits number which you set it before to secure your Shopee Seller balance.
5 - If he happens to have it, then he is able to access into your Shopee Seller balance and request a draw out. And this will Shopee will take 1-3 days to proceed and a notification will be sent to you.

Say now, is it so easy for hacker to hack into your account just to get RM8k? Why not he go and access into others with higher value?
And, could it be you or bf/gf did a withdraw and you don't know or forgotten?