I've spent some time using low focus mode to test it... but I just can't find out why it happens... sometimes the mined block doesn't drop...
tested it out on a vein of 4 diamonds and only got 3... but it happens on almost everything, didn't see it happening on nikolite, lapis and redstone... so stuff that drops in "bunches"...
Also not sure if it happens on stone or not... usually don't look at it when doing so... but love the new mods, the new low focus way makes the diamond drill obsolete... if only it resulted in a 100% yield, so that's why i still mine diamonds with the old drill