The background option for ADL is not passed to the server as noted in MONGOSH-932 when using the collection aggregate helper:

db.coll.aggregate([...], { background: true })

Taking a look at the code, it seems that AggregateOperation.execute only sets a very specific set of options on the command object before passing that to the server. This should at least include background. As outlined in MONGOSH-932 an upcoming change by MHOUSE-1267 will require us to also support further options at least from the shell side.