How to fulfill multiple incomplete series with zeros

I have multiple series with incomplete data, and I want to fill each series with zeros.
I can achieve this for a single series by appending OR on() vector(0) to the end of the PromQL query. However, this method doesn’t work when the query returns multiple series.
Logically, this could be achieved with an ** OR on() group_left vector(0) query, but “no grouping is allowed for set operations.”
Does anyone have any advice on how to accomplish the fulfillment?

Also, I’m interested in the root cause of the limitation on grouping for set operations.
An in-depth explanation would be greatly appreciated.