Okay - I have repeated the test with the same lens (FA 43mm Ltd), but after applying AF fine adjustment.
First photo - showing no FF (ISO 100, f1.9, 0.5 sec, at approx a few inches further from minimum focus distance) - EV 2.8:
Second photo - showing FF (same as above, but 0.8 sec) EV2.2:
As you can see, adjusting for the slight backfocus raises the EV level by which point the camera starts to FF (previously it was around EV1, now it's around EV2.5)
The batteries are spaced about 5mm apart in distance from lens, but the left one is slightly further apart from the centre than the right one (blame it on my butter fingers).