I tried a similar scenario to yours, and the Search/Filter operation worked fine - you can download the sample project from this link.
If you remove the third condition in your search/filter expression, do you see any results?
Search( Filter( 'Group Project Tasks', ParentID = BrowseGallery1.Selected.ID), ActionSearch.Text, "Title")
And if you use the other expression (with the 'If' call as the first element in the expression), do you see any results?
The answers to your questins are "Yes" and "Yes".
It works if I dont include this statement ParentID = BrowsGallery1.Selected.ID
As soon as I add statement above as a first statement or second it stopes showing me the result.
Let me know if you have any thoughts.