Multiple DEBs provide virtual package, which is listed as Depends
I expected that when I try to install this package without having explicitly specified which my-foo virtual flavor to use, I should likewise be prompted for a selection.
apt provides no such mechanism. I'm not sure what more can be said about this. You're expecting something that does not exist. There is no code in apt to provide such a prompt.
As @James S. pointed out, it will prefer the first non-virtual alternative if alternatives are given and all dependencies would be met by its selection. Otherwise, the dependency resolver will find a solution without prompting.