Using Krita 4.1.5 and 4.1.7 for Windows 10 x64, I find that if I attach one of the brush smoothing options to a toolbar, it does not show up again on restart of Krita. In addition, the buttons for brush smoothing modes do not even exist in the "Settings->Configure Toolbars" menu until a document has been opened, and the missing buttons do not reload until the toolbar is reconfigured with a document open. I expect the latter is most likely causing the former.

Yes, that's fixed in master.