I was just about to start this thread and saw yours. Have you solved it? Is there security on your API, if so, if you remove it, is there still an issue in the Android App?
Mine also works fine on the Web, but fails in the Android App. Hope to test it on the apple app soon.
Out of curiosity, what does yours do?
Mines uses a custom created API endpoint on my WP site and an uploaded python script inside the Custom GPT that work together to encode and decode messages. Works fine on web, not on Android App.
Check it here: ChatGPT - Sim Cypher v2 - Secret Messages by Veedence.co.uk