Right, like I said: crazy. Anything production with certain other clouds must be multi-AZ. Both reinforced by culture and technical constraints. Sometimes BCDR/contract audits [zones chosen by a third party at random].
The disconnect case was simple: breakage was as expected. The island was lost until we drew it on the map again. Things got really interesting when it was a full power-down and back on.
Were the docs/tooling up to date? Tough bet. Much easier to fix BGP or whatever.
IAD datacenters have forever been the place where Amazon software developers implement services first (well before AWS was a thing).
Multi-AZ support often comes second (more than you think; Amazon is a pragmatic company), and not every service is easy to make TRULY multi-AZ.
And then other services depend on those services, and may also fall into the same trap.
...and so much of the tech/architectural debt gets concentrated into a single region.