The session has a default 'incoming capacity' of 1MB, which actually has no effect by default and leads to unexpected behaviour while changing other configuration. It should be removed.
The value is used when the engine decides what incoming session window to set on Flow frames, but only if a specific frame size is configured for the transport, which it isn't by default. If a specific frame size is set, then any session capacity really needs to be set based on it in order to be appropriate, the default being fixed as it is leads to unexpectedly low window sizes in many cases which will impact performance in unexpected ways. Its confusing that setting frame size results in completely different behaviour than before for another thing (window size) which perhaps hasn't been configured at all, and the existing behaviour is also such that you cant turn it off.