I have a relay hooked up to an input tube, and I am expecting it to fill the buffer (in the gui), but it is not. Not really sure why...

Assuming there is no other place for the output to send items... the buffer should fill shouldn't it?

Rendering issue?

If you put something into a relay that has nowhere to go( full , no inventory accepts that item) it gets immediately backstuffed because it tries to eject it anyway.

from the 9 inventory slots the relay has, it is allowed transfer a stack from there into its internal buffer, as long as the buffer is empty. So one stack will turn 'invisible' at max. The buffer will only inject when there's a valid destination (either no block behind it, or an inventory with space, or a tube with a valid inventory at the end).

The given situation, is that also the case for you?

Yeah, I guess the internal buffer threw me off, I was expecting the internal buffer to be the 9 slots, visible.

I guess we can close it then :)

Well this is not entirely true. consider relay in setup where you want use like hopper, but when internal buffer is stuffed the block will not accept more items from tube network even if there are still free slots in inventory in it. The red power version this was possible. The output side when stuffed not output more items to aj inventory or pipe. The input sides are hover accepting items when gui slots are free. this block is kind of congestion of filter attached to chest(buffer).