Web Activity error on POST request - Data Factory

Question

I am trying to make a POST request to an external API using the 'REST' linked service as a source dataset of the data flow. I have a complex json array to pass but as a starting point, I am using the following json:

//sample json body
{"operator": "","operands": "","type": "expression"}

The error:

The same payload works well using Postman. Also, the same dataset works well when I change the API endpoint to a GET operation. The definition of data flow using POST is as follows:

I have also tried adding Content-Length header and used the size of the json both escaped and un-escaped, but I am getting a different error for both requests:

I am using self hosted IR using Azure managed VNet and there are no connectivity or firewall issues talking to external APIs.

Is it possible that it is due to the following bug: https://learn.microsoft.com/en-us/azure/data-factory/control-flow-web-activity#type-properties