This is now fixed, a build for the API backing that call was just pushed out.
And because programmer love some behind the scenes info porn, here goes:
The reason the user page isn't live yet for normal users on Stack Overflow is that we're still making some performance optimizations around the changes. One of the expensive things the profile has is the Accounts tab and panel (we call the sections on the summary tab "panels").
Originally, we had/have data for the accounts/users aggregated up to the network level in a single database, but not all of the data, namely: badges, and last visited and joined dates were missing. This means the accounts tab itself has to go visit various site databases and do a measurable amount of work. Kevin added badges to the aggregator which means everything we need for the panel could skip that per-site database user query, everything we needed was in the network database.
Further optimizing this, we only need to return the top 5 accounts/users + a count of total accounts/users (the count was added after we realized showing 5 was a glitch, and we only need 5 because the accounts tab shows more data which needs another API call anyway). However, this is a JSON API of simple data. And... unless you build that service with the new code (oops, my bad), that property won't be on the result...and the client deserailizing it into a class gets a 0 (default) on that property...resulting in what you saw, a 0 account list.