The question as to whether this occurs only in crossed combinations, or more generally is a valid one. If it's only in combinations that are rarely used the OP should live with it, and we might remind him not to ride in crossed combinations. But if it's more chronic, or chronic enough to become a nuisance it can be fixed.

In my 45 years riding bikes, including those in the dark ages when they were far less reliable than today, I've never owned, nor would have tolerated, a bike that routinely dropped chains backpedaling. I'm not saying a chain never came off backpedaling, only that is was a very rare event, well within my low range of tolerance for mechanical issues.