It could be a CSS specific problem. Can you try login with Administrator@pega.com and check the said behavior by doing private check out of the section pxRowEditing and making the field pyCommonName mandatory.
Ideally you should see the red exclamation mark against the field .Please share the result
When you say 'required' here - do you intend that the PageList has 'at least one entry in it?' - I'm not clear in my own head what 'required' means in terms of non-scalar properties.....(does an 'empty' PageList count as being 'filled-in' ?)
You can do this easily in Post Activity of the flow action, where you check for length of page list is greater than 0. Then Set Error Message on Page if length is 0. That way you will be able to force the user to add a row.