Yes, a circular orbit has an Orbital Eccentricity of 0.
I had a look in eddb.io at a few other bodies with circular orbits and they all have different Arguments of Periapsis.
Merope 2 a, Argument of Periapsis = 37.41º
Maia B 2 b, Argument of Periapsis = 46.11º
Diso 4 a, Argument of Periapsis = 292.98º
I think I know why this is. When Frontier pressed the button and set the galaxy in motion it was necessary to be able to describe both the bodies' orbits and their initial positions. It doesn't matter that most of the galaxy is procedurally generated. The Stellar Forge describes the orbit of every body and its position at Elite Dangerous time zero allowing current positions and movement can be calculated correctly whenever an instance for the system is created.
In order to find the position a body in it's orbit we use the True Anomaly which is an angle from the Argument of Periapsis.
https://en.wikipedia.org/wiki/Orbital_elements
Because a circular orbit has no periapsis we instead use the
Argument of Latitude. This is the angle from the Ascending Node and this is the sum of the Argument of Periapsis and the True Anomaly, and this is true no matter the Eccentricity of the orbit.
Argument of Latitude = Argument of Periapsis + True Anomaly
From a programming point of view you want to be able to use the data structures and methods for calculating the position of all of your celestial bodies. For non-circular orbits the Argument of Periapsis and True Anomaly are vital information and need separate variables. But for circular orbits we need only need to describe one angle, the Argument of Latitude.
We have two ways of doing this for circular orbits; we can store the Argument of Latitude in either our variable for the True Anomaly or Argument of Periapsis and set the other to zero. This way we can use exactly the same method for calculating the position of our body.
Remember that for any orbit, of any Eccentricity:
Argument of Latitude = Argument of Periapsis + True Anomaly
I believe Frontier chose to describe circular orbits by setting the True Anomaly to zero and used the Argument of Periapsis to describe Argument of Latitude:
Argument of Latitude = Argument of Periapsis + 0 (True Anomaly)
Argument of Latitude = Argument of Periapsis
When the system map describes a body as having a circular orbit (Eccentricty =0) and an Argument of Periapsis of 138.32º what we are in fact seeing is the variable used to describe the initial position of that body at Elite Dangerous time zero.
It isn't a true property of a circular orbital system, rather it's an artefact of the initial state description of the system.
Without a better explanation I suggest discounting the Argument of Periapsis as being meaningful for our investigation where it involves bodies with circular orbits such as Merope 5 c. That it appears similar to the angle in the UP message is a coincidence. If this turns out not be the case then FD have some serious explaining to do.
