What is going wrong?
When background: true is specified as an option to aggregate, the mongosh shell swallows it and doesn't include it in the generated aggregate command that's sent to the server. This doesn't happen with the old mongo shell.
How could an engineer replicate the issue you’re reporting?
Run a command such as the following:
What do you expect to happen?
The aggregate command sent to the server should include background: true.
What do you observe is happening?
The command sent to the server omits background: true.
Any additional information that may be useful to include.