I currently have a lvl 13 Light Cleric with the Observant Feat. With 20 WIS and proficiency, his regular perception checks are +10, and his passive should be 25(base 10 +proficiency +modifier +5 bonus from Observant). However, his passive perception shows up as 30 for some reason. The only items he has right now that would affect his perception is the Sentinel Shield but that just gives him advantage. How did this happen?

Advantage on checks that include a particular skill automatically translates as a +5 bonus to the Passive Score for that skill as well.

Passive Checks (https://www.dndbeyond.com/sources/basic-rules/using-ability-scores#PassiveChecks)

A passive check is a special kind of ability check that doesn't involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster.

Here's how to determine a character's total for a passive check:

10 + all modifiers that normally apply to the check

If the character has advantage on the check, add 5. For disadvantage, subtract 5. The game refers to a passive check total as a score.

For example, if a 1st-level character has a Wisdom of 15 and proficiency in Perception, he or she has a passive Wisdom (Perception) score of 14.

The rules on hiding in the “Dexterity” section below rely on passive checks, as do the exploration rules.