Boudica is going to DoW on me next turn. I don't understand how this is possible since we have a peace treaty with 5 turns remaining.

The interesting part is that you have 2 active Peace Treaties. One ends on this turn allowing you or Boudi to DoW.

I'm guessing it's a bug... don't ever recall seeing more than one peace treaty.

If someone becomes a vassal it cancels peace treaties - maybe that is what is happening.

If I recall correctly I asked Boudica for her world map during an existing peace treaty so that I could extend the peace treaty and avoid war with her.
Nobody became a vassal at that time.

I guess you can't extend the peace treaty while one is currently active then.

if the AI already has enough on their hands and you demand their world map, causing a 10 turn peace treaty, can they still attack you? because that happened to me

I guess you can't extend the peace treaty while one is currently active then.

I'm fairly sure this is true