There's a chance that the out-of-order messages bug is related to tests of iCloud Messages. It could be that they're testing iCloud Messages for a subset of users. For these users, they could be pushing messages to the cloud for each individual device, and then reading them back from the cloud (but still not syncing the messages across devices). If there was a bug in that logic, we would expect it to affect both iOS and macOS.
The bug only affected some users on some devices. For me, it affected my iPad, but not either of my macs nor my iPhone. A friend of mine had it on his iPhone, but no other device. Another friend only had it on his Mac. So it could be that one of each of our devices was in the test group.
With their continued work on iCloud Messages, this seems like a reasonable explanation for the bug, although it could be entirely unrelated