Hi, I've just seen this interesting post looking for some other stuff. I am in office on work laptop and I just imagine it....so I may be saying BS
In order to best understand this phenomenon, we should look also (simultaneously) at Orbit MFD.
RAnm (in degrees) is defined as True anomaly of Ref. Axis = the angle between direction of the Ref. Axis and direction of your ship's Periapsis.
This can be best visualized if you choose in SyncOrbit MFD as intersection point (MOD button) your ship's Periapsis (Sh Periapsis) - in this case RAnm = 0 deg. If you, similarly choose intersection Sh Apoapsis then RAnm becomes 180 deg.
IF your prograde burn happened anywhere else than Periapsis, in your current orbit, then your periapsis might move around and the 28 degrees setting is keeping your Ref. Axis "following" the Periapsis.
If you do this burn exactly at your current orbit's Periapsis then I believe that the Ref. Axis will stay put.....or maybe I am wrong. When I arrive home I will give it a try.