@b_jonas I will have to doublecheck, but I think that in the Reflection story-arc of the MLP comic there is yet another weird detail I didn't notice before.

Basically, the ponies visit the "mirror world" at two different times: first, Celetia and Starswirl visit it in the past, then a thousand year later the mane six have to explore the same world to fix some problems.

The mirror world has "negative copies" of every character (same appearance, opposite personality). Yet, while Starswirl and Celestia meet the "Mirror Luna" and "Mirror Celestia".... we never see Mirror Starswirl nor Mirror Discord.

Only later when the mane six go to the mirror dimension they get to see the mirrored version of Discord.

Basically, you never see alternate Starswirl and alternate Discord at the same time... and to be fair you never see alternate Starswirl in the first place.