Form actions return code 400 in production

[Hejtmus]

I have deployed SvelteKit (2.8.1, but also older version share the character) app to Google Cloud Functions (which runs on Cloud Run) using node adapter, exported handler as a cloud function.

The problem

The server correctly handles GET requests, but every non-GET request fails, in dev mode it works like expected. I have modified the adapter code, to print all incoming requests, and also placed similar logging to server hooks.
When making POST request, for instance logging in, the request reaches the first log (that means the problem is not in Cloud Run), but the request doesn't reach the server hooks.

Fixing attempts:

• At first, I thought it's csrf, so I disabled it, same behavior.
• Next I tried setting environmental variables, both origin, as well as requested protocol and host, logged output of get_origin, it was correct, but still the same result.
• Also I tried suggestions from this discussion, no luck.

Does anyone have idea what am I doing wrong? I can provide more info if needed.