I do not think that the Transparent Authentication is causing the issue. This would only break POST requests if the POST requests comes in when a new authentication redirect is required, and otherwise it would work fine. Can you verify that you are using the Authentication Server? This is only used in specific scenarios and if you are not using it we can directly exclude this as a root cause for the issue.
To do so check if on User Management -> Authentication Server the Listener Port 9094 is activated. If it is, please check the Mappings you have configured on User Management -> Policy Management -> Web Mapping and verify if any of the User- and Group-Mapping has the setting "Extract user/group information from" set to "Transparent Authentication".
If it is not you are not using the Transparent Authentication and are not affected by that common problem.
Besides that it is very difficult to say what is going wrong, I do not have an idea right out of my head. Capturing the traffic may probably help to identify an issue, can you provide some extra debug data here?
I would recommend to file a ticket with support to have a dedicated engineer working on your issue, as I think this is a very customer specific problem and the community won't see similar issues and so won't be able to assist.
Let me know if you have any questions.