is it normal for this model to lose all internal memory when unplugged, even for 30 seconds?
What do you mean by losing memory? Are all your channels and network settings gone after a power cycle? That would certainly not be normal. I can unplug my Stick for weeks or even months but everything is still there when I power it back up. If you mean having an open channel, and the Roku being on the home screen after a reboot, yes that's normal, but all your user settings should still be there.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.