Why is there water pooling only in the rear passenger seat area, while the roof and other seats are dry, with no water stains?

Check if the drainage hole is blocked.

I have the same issue with my car. The air conditioning sometimes works inconsistently, being cold at times and not at others. Once, I noticed that the trash and waste paper next to the air conditioning vent under the seat were completely wet, and there was a puddle of water under the foam. After closely inspecting the roof and finding no leaks, I deduced that the air conditioning drain might be blocked, causing the water to seep out.

My car's air conditioning is having the same issue.

Sunroof leaking water