If searches aren’t confined to the chosen realm, the realm feature might as well be removed, it would be pointless.
If you switch the realm to All, it will stay that way. But I don’t think that should be the default for new users. I don’t want new users to see over 1,700 topics and just give up. By the way, if you do a search that doesn’t work, then switch to all, the search will immediately update with the correct results, you don’t have to retype the search.
The way realms work is described in the documentation page for the Help wizard itself, and I will also include that at the beginning of the tutorial so that hopefully most users are aware of it.