RFC 3927 is the specification for how it should be implemented. It has a section 2.1. Link-Local Address Selection which specifies the usage of a RNG and recommends that the RNG should be seeded using the same initial value (e.g. a MAC address) every time.
The pseudo-random number generation algorithm MUST be chosen so that
different hosts do not generate the same sequence of numbers. If the
host has access to persistent information that is different for each
host, such as its IEEE 802 MAC address, then the pseudo-random number
generator SHOULD be seeded using a value derived from this
information. This means that even without using any other persistent
storage, a host will usually select the same IPv4 Link-Local address
each time it is booted, which can be convenient for debugging and
other operational reasons. Seeding the pseudo-random number