It is said that Sriranganathan himself has accepted Mamunigal as his preceptor. If so, why do we not celebrate Manuni Tirunakshatram? Adiyen is Vadakalai ayyangar; posing this query here as even after asking many Vidwan’s, have not received a satisfactory response. Moreover, have asked this query to understand why these differences in tradition exist.

There is no harm in celebrating the Tirunakshatram of Manavalamamunigal as he is also a Sri Vaishnava Acharya, but each tradition must celebrate their respective Acharya-s Tirunakshatram, which itself is sufficient.
Does Perumal even have an Acharya?
“peethakavADai pirAnAl piramakuruvAyirundu” (‘piramakuru’ means ‘Prathama Guru’). Perumal is the prathamAchArya, the foremost Acharya; hence, one must pause to think whether can there be any Acharya for Perumal.
Saying that someone is Perumal’s Acharya, is nothing but a metaphor. In our lineage of Acharya-s (Guruparampara), Perumal is the First Acharya. While on the subject of Perumal, Svami Desikan says “He alone is the foremost Acharya who does not have an Acharya”. Moreover, an Acharya grants knowledge to a disciple who lacks knowledge; But Perumal is Omniscient (sarvagnya), and he is not a disciple lacking knowledge. When it is said that someone can be an Acharya of Perumal, it is an act, a performance – this is like how Perumal during his incarnations as Srirama and Srikrishna assumed the role of disciple. Given this, referring one to be an Acharya of Perumal can only be a metaphor, a figure of speech. Perumal himself has not come down and pointed someone as his Acharya. Not just us, everyone in the sampradayam say this is a metaphor.
Based on this perspective, Periyaperumal has said that Svami Desikan is the “Srivedanta Acharya”; regardless of the differences in the tradition, Sriranganthan’s divine wish that Svami Desikan is the Acharya for the Vedanta-s.
The traditional differences arise due to the differences in the expositions. Each Acharya-s’ exposition follows different interpretations. For one, their respective Acharya’s commentary is correct. Over time, the differences in doctrine turned out as traditional differences.
Notes:
Not sure if it is right to say, just because he is mentioned as an Acharya of Perumal, we must celebrate his Tirunakshatram; As per the Ramayana, Rama’s Acharya was sage Vashistha and as per Mahabharata, Krishna’s Acharya was Sandipini. we do not celebrate their respective Tirunakshatram-s. So, no need to celebrate one’s Tirunakshatram because one is said to be Perumal’s Acharya.