I've seen quite a few compilation albums with no lineup, even compilations from larger bands. Is this deliberate or is it just because the information is missing/yet to be added? Thanks.

A lack of information is almost never deliberate. For the one collaboration album I own though there are no liner notes about the members of each band, of which there are five. I'd imagine a lot of collaboration albums are similar, or people are just lazy.

It's not deliberate at all. If the information is available, then it should be added. But no guesswork based on the time period it's from or anything like that. The info has to be verified to be added.

Do specify, though. It should look something like this or this.

Adding (full) comp lineups is a relatively young feature, so that's probably why there's still gaps.