Also called the temporal line. What could cause this?

I would guess you are just developing your temporalis muscle from chewing exercises.
Why would that make the bone edge feel different? The temporal ridge is at the edge of the temporalis.
Try putting your hand where you feel the bone changes and then clenching your jaw a few times. Do you feel the muscle moving or is it definitely just a bone change?
No it doesn't feel any different when I clench my jaw
The muscle is probably drawing your attention to it more depending on how distal it is from the joint .
its just fat loss around the area/skin tightening around your face
its just fat loss around the area/skin tightening around your face
Neat!
Yes, mine has become prominent too. It seems expectable considering the rough direction of forces involved:

As for fat loss, it's not that unless you have lost significant weight. Fat distribution doesn't just spontaneously change.