Post from PouchDB to CouchDB via IIS ARR reverse proxy ends with 502.3 (Bad Gateway)

When I make a POST request to IIS reverse proxy which redirects to CouchDB without any data, I get 400 Bad Request as a response. This is expected and correct.

However, when I make the same POST with POST data included, I get 502.3 Bad Gateway 0x80072efe ERROR_INTERNET_CONNECTION_ABORTED. This is not expected. I should get 400 Bad Request back.

Here is log comparison of the two calls. First is POST with data, second is POST without data.

https://www.diffchecker.com/vkwnbear

Do you know why this happens?

Background: PouchDB calls _bulk_get on CouchDB through IIS ARR reverse proxy. _bulk_get does not exist and should return 400 Bad Request which is then handled by PouchDB.