There is a data buffer about 5 Mb even with your data turned off. For example, when your plan renews, the data tracker will show 5 Mb usage even though no data is used. It is not an ongoing data leakage.
Could you please provide more details? What type of phone? Where are you looking at the data usage...on the phone settings or Public Mobile site?
Thanks and welcome to Public Mobile 😊
I do know that the data usage showing on the self-serve landing page CAN BE off a little bit.
So there is a kind of buffer which seems to be applied as put forth by @Triguy