Well, I think renko bar creation should have been based on comparing low/high instead of close price. However I think the issue here is a bit different.
From manual:
The bar function is automatically called whenever a new price quote arrives or a price tick is read from a historical file.
'tick' in above case is 1 minute bars in t6 file. Sometimes, (e.g. news event, price gap) a 1M 'tick' can be several times bigger than renko bar size. As a result, you would expect several renko bars being created. But bar() function can only return "4" (closing custom bar) once per 'tick' and generates only 1 renko bar instead.
