Yes, Plex will, in the right conditions, create a “Various Artist” artist when it is able to. Maybe it found less ambiguous information in the smaller library and, as you suggested, you got lucky.
I gave up (never started, really) trying to figure out why Plex does what it does when my tags were incomplete. Maybe the scanner found different clues in each situation and, trying to make the best guess it could, came up with a different result. I just took it as motivation to get my tags in good order.
Artists that only appear on VA albums will not get their own Artist Detail page in Plex. This is a long-standing feature request. But searching for an individual artist on a VA album does work.
Here’s an example. In my library, this artist only appears on a single track on a VA box set:
As noted in the How-To article I linked to earlier (and the Help articles also emphasize), if you’re going to use the “Prefer local metadata” option, your tags need to be very accurate. The How-To also lists the specific tags Plex needs at a bare minimum (look for “Necessary Tags”).
I’ve found Plex to be very reliable as long as they are. When Plex does something odd with an album, 99% of the time it’s because of an error in my tags.
