If you have an overdraft with your bank, it is likely included in your available balance.

This is because, with Open banking, the bank sends the information directly - when they do this they show the available balance to include your overdraft. At the moment we cannot change this feature. 😕

So, what you need to do to make sure Plum doesn't touch it is go to

Brain > Overdraft savings > toggle OFF.

Finally, you might want to deactivate all the rules apart from the Automatic as they are not dependent on your balance but on bank transaction or other factors such as the weather, your Pay Day, your Round-Ups, etc.