Well, this took me a few hours to figure out, but there is a bug in the parts processing logic, but there is also a workaround.

I have a very small admin-pages welcome.php file. It is copied from the piklist demo file, but with nearly all the HTML removed (except one div with one H1 line) However, I left all of the CSS style directives in. The only thing in the header is the Page directive.
The page does not show up anywhere. I eventually found that the position was set to absolute; (in the page->data that also has title, tab, subtab etc.).
I traced it to the code that reads in the parts file and tries to find the header directives. Because the file was so short, and I did not have a Position: directive in the header, the 8k that is read in includes the CSS syntax. The result is the regex matching a line in the style declarations that had position: absolute;

I have verified it works if I remove the css code from the admin-pages parts file.

I have verified it works ok if I put in Position: before in the header. So that is the workaround for now.