They stack
Fifth edition largely did away with things that don't stack with eachothereach other. If two different things both grant a bonus to something (and those bonuses are numerical - not like "Advantage" or "Resistance" which either you have or you don't) they stack unless they say they don't.
Partly because of this, first party material (items, feats, etc.) for 5e tends not to hand out many bonuses to ability checks, saving throws, and attack rolls.
In the case of passive perception, though, it would be very difficult to break the game. That would require that some aspect of the game be balanced around characters not having a high passive perception score, or perhaps have some mechanic that allows a character to leverage their passive perception score into bonuses or advantages in other situations. Neither of those things exist as mechanics. It seems to me that the designers are willing to loosen up and hand out more numerical bonuses to this particular value as a result.