BUG REPORT : Power Query Mashup Engine April and May version runs forever

Source table size less than 300K rows, accessing data via an on-prem Gateway, from Power BI Desktop April & now May version.

I have a similar situation but it is even worse as the Mashup Engine threads water forever in the last step of a reference chain of intermediate queries with Enable Load = OFF. This is what it looks like:

query_0 > query_1 > query_2 > query_3 > final_query

A few days ago, with the same M code, all was working and being processed at an acceptable speed. But after a Save As and a machine reboot (using the April version), suddenly query_2 and query_3 would take the 3+ hours to complete! Same happens after updating Power BI to the May version.

I log in today, see that finally query_0, query_1, query_2, query_3 have all completed but there still is a question mark next to final_query. I click on final-query and it goes on and on forever. And the code of the final query is this simple:

let 

 Source = query_3 

in 

 Source

It is just a reference to the previous query, which has completed. And when looking at the Task Manager, the Mashup Engine is showing Very high power usage using > 11% CPU and 2GB of RAM!! What on Earth is it doing? All the code says is pull the data from the previous query into a table and load it into the model! Why is it stalling when all previous queries have completed successfully?