I guess I lucked out when applying for ACA. Don't remember any requirements.
I'll be applying for Medicare in February. I'll come back with an update.
Hopefully the healthcare marketplace has removed the requirement for naturalized citizens to prove it. The annoying thing when I applied was that there was a place to enter the Naturalization number off the certificate but the format acceptable on the screen was different to the format on my and my wife's certificates so we had to download copies for manual review which then took a few days.
My Medicare application was via telephone interview with the FBU in the US London embassy and the staff there may be more aware of the likelihood that expat US Citizens may have been Naturalized.