Di2 Issue

Ultegra Di2 12 speed:

Get the bike out of its winter hibernation. Both shifters completely dead, but they are 20 months old so no surprise there, replace the batteries. Still completely dead. Test the original batteries and they are obviously pretty low but still have a discernible charge. Shimano software appears to recognise that shifters are there but the buttons do nothing. I update the derailleur software. Take the bike to my bike shop, they confirm that nether shifter is responding to button presses, they update everything, but still zilch, they say they will send them back under warranty but will take a few days to do it.

Half an hour later they say  phone to say they were fiddling a bit more & now the LH shifter is working, but not the right. Fast forward a few days & I get the message that when they came to take the RH shifter off they found it was working & now both seem to be working ok (haven't picked the bike up yet)  

So has anybody got any experience of this? I can't work out how both shifters can stop working with no obvious single point of failure & then spontaneously start working again. Its getting close-ish to the end of the warranty period & obvs I want reliable shifters. Any ideas?