[AI Bug] Monsters not targeting other AI players, being super-aggressive on players.

not a bug, this behaviour is inherited from WoM - probably the AI is unable to avoid monsters so settlers are not targeted otherwise it would be unable to create new outposts / cities, so it must be intentional

In my games the AI Pioneers get slaughtered by monsters all the time, so it's not a feature.

I watched two different AI stacks of Scouts + Pioneers become monster meat in 0.91. And I was quite thankful. It let my puny little hero sneak by those monster strongholds without getting molested.

It's not consistent, I've seen the monsters go after non-pioneer stacks and occasionally other AI, but often they just get ignored and they suicide on my units instead.

I think there is an obvious difference in valuing between at least certain AI units and the human units, that isn't based on any status I can tell.

It's possible I could be wrong, but it doesn't seem that way right now.  It's also possible this is situational. 

At the very least, the AI does need to build fewer pioneers, it's ridiculous how many I kill when I'm at war.

Wandering monsters are definitely aggressive toward AI players in .91.

I do agree the AI gets to do some stupid moves once in a while due to monsters ignoring theyre pioneers, or scouts, or whatnot (especially the bigger monsters), But I did see the AI get attacked alot of times by natural monsters, they just "Keep Trying" to move stupidly close to monsters with puny units, so you WILL see the pioneer having luckily sneaked past 2 great drakes, 1 wildlands boss, a trillion wildlings, 3 death demons and a cave bear before going up to build an outpost in your face...

Sincerely
~ Kongdej

so pioneers are everywhere because they are so numerous, not because monsters leave them alone ? I did slain an army composed only of pioneers

Something like that

Sincerely
~ Kongdej