Hi Ksnfme,
Yes, I do find that peculiar.

I have used the FS-A8S many times and it should work just fine.
First of alll, and I'm sure this isn't your problemm, but I have to mention it, that of course you need to make sure the iBus receiver is plugged into the correct set of pins on the TCB (ie, those pins on the top of the board, and not on the bottom).
Secondly, the only other time I've heard someone have a problem with the FS-A8S
was in this thread, and I'm linking to the post that had his solution.
In short, the receiver may be in SBus mode, and it's not entirely clear how to switch it back to iBus. I encourage you to read the post, but in short, in addition to having iBus selected on your transmitter, you can then toggle the protocol on the receiver side by pressing and holding the bind button the receiver for 2 seconds after it is powered up and already bound to the transmitter. Give that a try and see if it works for you, maybe the receiver is still in SBus mode and that will swap it back to iBus.