[How to select multiple objects in the viewport?]

Thanks for the tip, but that caused the issue to start happening again. I think I must have tried using this shortcut previously (perhaps to hide / unhide something). Is there a way to turn this feature off again, without just resorting to restarting the app?

Edit: found it. Choose Pick Current Layer (H), rather than Pick Layer Add, at bottom of Windows > Panels. 

[How to select multiple objects in the viewport?]

I'm having the same issue as Pior - selecting objects in the viewport with Quick Pick just keeps adding to the selection. Selecting in the Outliner works as expected. It's only recently started doing this for me. I'm on the latest version (2023.28), which I installed a couple of days ago.

Edit: restarting 3DC fixed the issue.