Failure mode - running out of fuel.
What do you think running out of fuel whilst having 70ishkg on board looks like ... how does in stop?
Quickly? Instantly? - or with a lot of re-igniting as more of the 'unusable fuel' becoming useable now and then?
If an 'unuseable quantity' is so relatively high then reaching that 'unusable' amount will not be a clear and instantaneous event but a drawn out process of intermitancy. Similar to what the witnesses describe.
If the unuseable quantity were 1 kg then the engine stoppage would then be much more clean cut.
Taking a long time and being very messy would be very disconcerting for the pilot and could cause a very confusing and awkward cockpit environment whilst trying to make sense of it all. (part of the inherent downside of a twin which gets dismissed here thoughtlessly)
I don't think certification requirments deal with such considerations, and since helicopters are effectively designed by regulators it would be unexpected if the manufacturer's designer would bother thinking about such non-mandatory aspects of design.
Has anyone tested the running out of fuel process in this type?
I doubt it.
Should they? OF COURSE!!
or is everyone happy to just go on thinking it's all very simple and easy: One engine just runs down you then have 2 mins to land in an orderly fashion, God is a smiley bloke sitting on a cloud, girls enjoy head, the cheque really is in the post, your NAA is there to make things safer ?