Thanks for all of the responses.
We recently migrated from v21, so that is consistent with Andy's observation.
Do any of the programmers know if this problem would exist if we downgraded to Deluxe or went back to 21?
Single layer blocks are pretty limiting. Just like sticking pictures on your drawing. Multi-layer blocks allow a real modelling approach, more consistent with BIM etc. For example, we have blocks of building columns. Insert one column into a model space of a particular storey and it appears on the column plan as well as the floor plan, as well as the floor plan of the floor above (showing loadbearing structure under). Changes reflect in all viewports simultaneously.
Now in TC2018 the columns can appear in only some viewports, depending on what layer is selected when blocks are dragged and dropped.
Could we request to have the blocks behaving like the manual says they will (or at the very least have a temporary note in the manual saying that blocks may insert in the active layer and it is recommended to switch to layer 0 before inserting blocks)?
Thanks again for the fast responses.