I don't think this is new, just now easily noticable with the recent changes.

After completely building and building, marine or alien, sometimes they building is
only at about 99% health and has to be healed or welded to 100%.
doesn't always happen. Seems to be something about the number of people building said building.

The so-called 99% health bug has actually existed for pretty long time, it's just now being noticed, as the poster suggested, as of 3.0 Final because only structures with 100% health show full rings (this was changed at the same time as "only structures with 0% health have no rings").

Todo: Check exactly which structures are affected (are Hives?), and if it happens in both Win32 and Linux settings. It definetly frequently occurs for Alien structures, which then just heal themselves with their auto-regen, but I'm sure I've seen this with Marine structures too.