Bit confused, how do you sell the individual game and soundtrack on sale separately but not the bundle?
Superliminal for all.

It's not just Superliminal as The Quest is like this as well. The standard version of the game has been on sale a few times as well as its expansion, but never the deluxe version of the game that includes the expansion. I'm sure that there's probably a few others games that are like this.

I don't like that as well, one example comes to mind is The Evil Within OST, it's not included into bundle and I think it was on sale only once since the release, for 50%:
https://www.gog.com/game/the_evil_within_soundtrack

That is by design ackshually.
It's a custom bundle for gog to simulate the 'bundle' that steam puts together.
The bundle on steam right now is 'on sale', 21% off for.. $14.98, which is the every day price on gog.

Developer commented about it here:
https://www.gog.com/forum/the_quest/about_deluxe_edition_price/post5

That bundle is always there at a better deal than regular price, for people to pick it up while there is no sales.
It is no different than buying The Quest + The Quest Soundtrack + The Quest - Islands of Ice and Fire.