Here's the most recent TSB on 'Tapping noise from engine', which supersedes all prior communication on this subject. It has additional information, but IMO in no way will make anyone feel happier about owning a 2018-20 Skyactiv-CD engine.
The most important new statement from Mazda in this revised TSB is that 'the switchable HLA has been modified for increased oil supply to reduce air bubbles in the oil passage'. So this certainly sounds like confirmation from the automaker that the factory switchable HLAs are not subject to 'failing', but instead were designed incorrectly by not providing for enough oil flow.
And so the new question question becomes, how many engines will eventually produce whatever set of conditions are required to create the low oil flow, air bubbles, and the noise? Just another episode in the Skyactiv CD horror show.