Calculator on MX Keys S does open the Calculator App however, it remains in the Background Making the button kinda useless.

On the Older Logitech N305 That I have it works as one would expect.
it opens the Calculator App and makes it Active so you can use it right away.

So I assume this might be a Bug and wanted to let you know about it.

Talking about my Old N305 Since that still works with the Unifying-Reciever I am looking for a Replacement that uses the Logi Bolt Reciever since that would provide a more secure connection.
However I could not find such a device among your more recent products
So I hope that in the Near future there might be a followup to the N305 as it has always been a very useful device in my opinion