Please note/ Please be reminded of

Let's see it this way: To 'note' something is a positive act to be carried out by the person you're addressing the phrase 'please note' to, therefore a passive form should not be used.

On the other hand, "please be reminded" bears the meaning of the first person (Subject) causing the person receiving the 'reminder' to be aware of something, therefore it's used in a passive way instead of an active way, as you probably wouldn't ask the recipient of the e-mail to remind him/herself sth by him/herself, it's you who's asking him/her to be aware of certain things...

Sorry, I've tried to illustrate my point but it's hard in a way, hope it's helpful.

A simple way to look at the situation is whether the audience/recipient is the subject of the verb (the person taking the action) or the object of the verb (the person impacted by the action).

The customer notes a problem. Since the customer is the subject, in imperative, you will say "Note..." or, politely, "Please note..." (Imperative, active voice)

On the other hand, someone has to remind the customer. In this case, the customer is the object of the verb. As a result, the imperative form is "Be reminded" or "Please be reminded..." (Imperative, passive voice)

Here are some other examples:
"Please contact..." The customer contacts you.
"Please return..." The customer returns something to you.
"Please accept..." The customer accepts something from you.
"Please be assured..." You assure the customer, and the customer is assured.
"Please be informed..." You inform the customer, and the customer is informed.