Just opened my first flow tub & was surprised to see no seal.
Anything else I can think of, whether I buy retail or via the web, has a seal underneath the screw top - coffee, powdered milk, capsules from health food shop etc.
Glass jars that don't have a seal, do have whats known as a tamper proof mechanism where a button pops up once the seal is broken.
So just the screw top lid on flow - is that how it should be?