Many reasons exist for your described problems. Miss matching der cage width with ring c-c spacing. Poor cable tension management (sounds like you've explored this). Der alignment WRT the rings. Expectation of cross chaining not rubbing. Cable friction (although ft systems are usually less sensitive to this then rear).

To find out if chain rub is actually the issue remove the ft der and see what happens. No rub then you did identify the problem. Chain rub is pretty easy to discover though. Andy.