Auto-fill shortcuts do nothing on Penzu.com

I don’t know how many sites this is true for, but at least on penzu.com, for some reason, the “auto-fill on page load” feature works fine, but when that’s turned off, the other Auto-fill methods do not work (keyboard shortcut, context menu, and vault popup).

Any idea why the page-load auto-fill works, but not these other methods?

Browser: Firefox 119
OS: MacOS