Perhaps your microphones are facing too far upward, so when you bow your head to your iPad, they receive sound directly from the next table instead of the tablet.
I have that problem with previous receiver cable. According to the measurements, I should have gotten a size “2” cable, which was too long and caused my HA to wobble on my auricle. I’ve got the size “0” now, and it is better.
See that post:
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Speaking of speech and noise signal from the same loudspeaker:
I think you are correct that automatic Spheric mode relies a bit on directional signals. However, I suppose that in the manual Spheric program, things would be different.