Ok, could you please post your app (remove of obfuscate any sensitive data if necessary)? You will need to zip it in order to post it as there is a bug on the Qlik Community site that prevents one from uploading the qvf files.
Take a look at the attached app (you'll have to unzip it). I've included category in the data and if I use Avg(Sales) with just WeekDay in the table, the formula still works since Qlik Sense calculates it on the context of the dimensions in the table. I've also added another table which also includes category and in which I've used the formula: Aggr(Avg(Sales),WeekDay). This shows the same average figures too.