Mist ogres, sneak attacks and archers, oh my!

[Mort]

Monsters dont have the regular uncanny dodge. They have the feat called "Monster Uncanny Dodge" which makes it so they can never be flatfooted.

Seems to be hardcoded.

[Jayde Moon]

Does Monster Uncanny Dodge resolve why I'm able to sneak attack these guys when attacking out of stealth or invis, but not otherwise?

For testing:

Critter:  Flayer Stargazers and Flayer Slingers

Hitting them from stealth or invis, they are not engaged in battle = Sneak attack

Hitting them not from stealth or invis, they are not engaged in battle = N/A, when they see me, they 'engage'

Hitting them not from stealth or invis, they are engaged in battle = No Sneak Attack

Hitting them after walking away from where they are fighting, turning a corner, stealthing, walking back to the fight = Sneak Attack.

Basically I can sneak attack, run around a corner, stealth, return and sneak attack.

Monster Uncanny Dodge explains this?

If not, is there anything that does?

Perhaps just bugginess?