I tested this too in ChatGPT and even with a very explicit prompt it still generated a checkerboard background instead of a true transparent PNG.
Prompt tested:
Create a single isolated centered flower icon. Output as a PNG with real alpha transparency. The background must be actually transparent with an alpha channel, not a checkerboard pattern and not any visible backdrop. Show only the flower, centered, with no scene, no floor, no wall, no shadow, no glow, no text and no extra elements. Leave a small margin of transparent space around the flower.
The result looked like transparency, but the checkerboard was baked into the image. So it was not a real alpha-channel transparent background.
So from my test, ChatGPT UI does not seem reliable for true transparent PNG output. For reliable results, I’d probably use a background-removal/editor step after generation or use the API where transparency can be controlled more explicitly.
@Bleatlock might have better solution for this?