Hmmm... interesting.. he's right, it's definitely not working as intended.
*First test with yellow belt on the input, output, and splitter
If input is one belt with one side of items, then it sends all properly to the priority output (plus belt balanced), but as soon as the input has items on both sides of one belt, the output gets muddled, being almost a near even split, with only slightly more going to the priority lane.
However... I tested it again with a red splitter instead, everything else the same, and it properly sent the full yellow belt to the priority lane. Makes me wonder if the logic of the yellow splitter is too slow to actually handle yellow belt?
Third test, added another half-full belt to the input, and upgraded the output lanes to red belt. About 2/3 of the items went to the priority lane, but still not the expected result (of a full red belt for priority output)
Fourth test, upgraded the splitter again to express and then I started getting... inconsistent results. Sometimes it worked perfectly, sending all of the input (still one and half yellow belt) to the priority output (red belt), but other times, it would mess up and do something similar to before. It seems to depend on the timing of when I attach the input to the splitter.
Gif Attached
...I spent over an hour trying to record and upload as gif with this comment typed up and ready to send, didn't even see that you had responded already... ;-;
The priority input was working fine though! At least.. as far as I could tell.. (with 2 fully compressed input belts and one output belt)