Select HIDESEL then SHOWHIDDEN behaviour

If I have some items selected and I...

Select HIDESEL NOPATTERN
Select NOPATTERN SHOWHIDDEN

The files that were selected before being hidden are no longer selected when unhidden. Just curious - Is this intended behaviour?

Yes, it is intended.

It prevents you accidentally doing things to files you had just unhidden and did not realise had been selected long ago in the past at the time they were hidden, and may be scrolled out of view.