Metal Ceiling Rust

Found rust on cheap metal ceiling panels and paint peeling off.
The IR image shows the cool duct and AC air causing cold
areas across the ceiling. The rust seemed to follow the same
path as the cool duct and AC air.

It appears to me that you may have air leakage behind the register where the duct boot passes through the ceiling. There isn’t a distinctive flair that we normally see from the supply air leaving the register.

You are correct. That is why I left by comments very general.

I think it was cheap flex duct laying on the metal ceiling tiles (seen in other photos),
plus as you said, air leaking from behind the vent connection.

Cold air, humid area of texas, tall ceiling, cheap metal tiles, Texas heat,
in and out life style, room near the front door… cheap duct and poor connection.

Condensation = Rust

Actually, the picture I posted is leaking behind the register grill, but not above the ceiling. Just want to keep things straight! The flares are more bold and only directional with the configuration of the register. Still it’s air leakage into the room, not above the ceiling.