I have dropped a Devpack Component into a Flex Item Container and it isn’t showing the Devpack Component in the Page Layout tab. It is shown in the WYSIWYG editor. I have added a separate Container (non-flex) to the page and added the Devpack Component and that is revealed in the Page Layout. See image below:
Hmmm, If you save the project, and restart Elements does it appear?
Yes it does 
I guess it is a bug with a workaround if it happens again. 