Canada

I'm saying this without any actual proof, but I strongly suspect the Mennonite outbreak is among a group of Old Colony Mennonites. Due to pressure from various governments, Old Colony Mennonites migrated from Ukraine to the Russian Empire, then to Canada (where some stayed), and then to Mexico; this left them with a cultural distrust of governments and government agencies (such as public health agencies). They settled in rural Mexico and avoided the government and public contact as much as possible, meaning that (among other things), they managed to avoid the great vaccine push of the 1950s and 1960s, whereas more mainstream communities were educated on the benefits of vaccines and got to see those benefits in their everyday lives. When Mexico's economy deteriorated in the 1980s and 1990s, some of those Old Colony Mennonites moved to Texas and others back to Canada, where they remain in under-vaccinated pockets.

I haven't heard anything about which specific branch of Mennonites are involved in the outbreak but, as I said, I suspect they're mostly Old Colony Mennonites.