Currently their order appear to change based on sales data or something, because more known / recent titles always show up at the front.

This is of course not a problem unless you somehow has a long list of games in a bundle and would like to go through them on separate days.

So it's largely an edge case, but I think we shouldn't have this edge case to begin with: please order them in a deterministic way?

Yup. If you own a certain bundle with a certain 59 pages of content, good luck browsing through it all. You'd either have to go through it all in one sitting or hope the ordering doesn't change much, lest you completely miss certain entries. Especially since none of it's added to your library automatically and they've added no option to do so despite saying they're aware it's a wanted feature.