As it turns out, Microsoft was planning to release build 10586.63 cumulative update to Windows 10 Mobile Insiders in the Fast Ring. However, during the publishing of the update an issue was discovered at the last minute halting the process.
Microsoft's Gabriel Aul took to Twitter to explain the situation:
"Sorry #WindowsInsiders for confusion on the new update for W10 Mobile today. During publishing of the .63 update, we detected a potential issue with staging on deployment servers. Since we were partially staged, some people were able to detect the update.We paused the update to investigate, and ensure that everything was 100% ready. We're now resuming the update and it will roll out to #WindowsInsiders in the Fast ring today."
The good news is, of course, that the update should go out to all Insiders today at some point. The problem detected was a distribution server one and not the build itself. In other words, there is no risk for those who have it installed or are starting to install the update.
As to what the update contains, precisely, we are still learning more. However, Aul did add to this by saying "...we're doing very targeted changes to fix specific issues at this point with Mobile." However, do not expect a changelog, per se: "Not a detailed list. Mostly upgrade fixes and other corner-case things per device model."
Finally, regarding our earlier report about the new firmware and Insiders who cannot get it: "No news to share on that yet".