The math that describes light in one reference frame is a mathematically perfect straight line. In a different reference frame the math that describes light is curved.
Just like a straight line in one coordinate system can be transformed into a curved line in another system.
There’s no absolute frame refrence in physics. We’re talking math theory here.
Light in an accelerating elevator is physics. Light in an anything is physics.
https://xkcd.com/435/
The math that describes light in one reference frame is a mathematically perfect straight line. In a different reference frame the math that describes light is curved.
Just like a straight line in one coordinate system can be transformed into a curved line in another system.