For years, when I'd select the "New Part" command...
...I'd get a dialog box asking me to select my Part template, then when I had done so a virtual Part would appear in the Assembly tree, and it would be "Fixed".
I'd have a green check-mark beside my cursor, but as soon as I clicked somewhere in the graphics area it would go away, and I'd be free to select to edit it, or do any other operation in my Assembly. I remember others here reporting different behavior, and being puzzled by it. Which brings us to today, when for the first time since upgrading to SW2019 I selected "New Part". I got the same dialog box asking me to select the Part, but after doing that I was surprised to get a dialog box asking me where I wanted to save the Part. I did that, and then when I returned to my Assembly I was surprised to discover that my new Part wasn't in my Assembly.
It turns out the option shown below had gotten checked. I de-selected it, and everything returned to the workflow I was accustomed to.
Maybe I'm the only one that wasn't aware of this, in which case I'll feel pretty foolish, but if not maybe it will help someone else who's run into the same situation, or who sees one of the behaviors I described above and would prefer the other one.