Either can be val;id as a metaphor, or as a literal description. There is no rule which confines one preposition to one meaning here.
One most often says that one
Sees X in a different light.
when one is speaking metaphorically, meaning that one perceives X in a different way, often due to some change of circumstance.
However, an artist or a photographer might speak of "seeing the subject in a different light" when refering to actual lighting conditions.
Under is more often used for actual lighting conditions, as in:
I saw her under a bright, direct light.
But when the light is described as "different" that somewhat suggests a metaphoric use. As this Google Ngram shows the form "in a different light" is much more common than "under a different light".
Even so, the use of "under may suggest a literal comparison, as in:
A colourist may be happy with his matched sample under a particular light, but when the user observes the standard and sample under a different light, he may be surprised to see distinct colour differences between them. [Principles of Colour and Appearance Measurement by Asim Kumar Roy Choudhury ( 2014 ) p 174)
But in some cases "under" is used with a metaphoric intention. For example in In-Between Identities: Signs of Islam in Contemporary American Writing (p 117) the author cites a rap verse 'Undder a Different Light" where the expression is clearly used in a metaphoric sense. Other such examples:
- Under a Watchful Eye: Self, Power, and Intimacy in Amazonia (2013, p 214) --
... rather than simply “created” in Amazonia, the conceptual framework advanced by Gell (1998) may help us to pursue these issues under a different light.
- By Monomoy Light: Nature and Healing in an Island Sanctuary by North T. Cairn (2000, p 26)
It's the specifics that trip you up — or , under a different light , illuminate the finer points of what it means to love.
- An Infinite God and a Father-Son God: The Theology of God by Kenan Osborne OFM (2019)
We will better appreciate the ancients if we lay aside the above flawed methodology and remember that they are singing in a different key, or—to change metaphors—that they are working under a different light.
- A Conversation Analysis Approach to French L2 Learning by
Clelia König (2019)
Narratives, however, have also been analyzed under a different light, namely regarding their intrinsic property of structuring the conversation in a specific way.
- A History of Theology by Yves Congar, Joseph Guthrie( 2019, p251)
The history of biblical doctrines and Christian doctrines materially have the same object as positive theology, but they view and attain that object under a different light and according to different criteria.
- The Tito–Stalin Split and Yugoslavia's Military Opening by -
Ivan Laković, Dmitar Tasić (2016, p145)
Chapter Seven: "Under a Different light"
- The Constitution Under Social Justice by Antonio Rosmini (2006, p. xxxvi)
In a manner of speaking, his sympathies for the Restoration should be looked to under a different light than those of the great conservatives.
- Historical Social Research [Issues 115-116] (2006, p. 127)
This puts the classical units of comparative history , especially national formations , under a different light , their hold on us becoming more obvious , and maybe more controllable.
- Justifying the Margins by Pierre Joris (2009 , p. 27)
prefer to consider it under a different light, the light of my own “nomadic poetics” which, I think, is better able to articulate the collage or assemblage aspect of the work with its syntactical disruptive modes.
- Puzzle Films: Complex Storytelling in Contemporary Cinema by Warren Buckland (2009, p. 103)
The fact that it appears in Leonard's mind after the killing of the wrong person puts the entire story under a different light, and we touch on the unreliability of the main character.
Hotbeds: Black-white Love in Novels from the United States by
Pia Thielmann (2004 p. 94)
The fog has a racially unifying component, but under a ifferent light, under closer scrutiny the white woman's race / ism will show.
- Is Scientific Knowledge Rational? by Halil Rahman Açar (2008, p. 14)
In the following pages, I will try to look at rationality under a different light and defend it.
Many additional examples could be cited. While a general search for the string "under a different light" seems to return about 90% literal and 10% metaphoric uses, a google books search excluding "source", "field" and "level" gives a much higher percentage of metaphoric uses. Excluding "Polymer", "plastic" and "dental" as well gives more than half of the uses clearly metaphoric.