Thanks, all.
I was thinking the issue was something to do with building the string for the confirmation message. The offer peace message was awfully crowded with full names for the first 4 properties. I had thought it was trying to build a string that was too long and couldn't fit in the fifth territory name.
It's definitely the max warscore thing. I replayed the war again yesterday, and the warscore never went over 100, even killing mexican troops and rebels. I needed 134 to acquire all the territories at once. There doesn't seem to be a way to avoid a another Mexican-American, I had intervened in the Texas revolt (at the beginning of the game), but didn't put in the war goals for Mexican Texas then. That still wouldn't have helped. I would have needed to Return Cores on Texas and New Mexico or Arizona (and maybe all 3) to reduce the total warscore needed below 100. California alone is worth 44, so the other states only total 90.
I doubt I'll retry that from the start.