Platform, device version and operating system
What you were expecting to happen, and what actually happened
I expect the delve during event to be quite exactly the same as the normal delves. So, I also expect the event delves to have stone blocks like the normal delves in that faction.
How often does this happen? When did it begin happening?
happened always during both delve events: on Tuesday and on the weekend
Steps to make it happen again
Play a delve event. Wonder where the stone blocks are.