Significant update, and good news. I have been in touch with ObDev via email, working with them to nail down bug reproduction, with good success. Although I don't know why ObDev isn't monitoring/participating here, I can at least confirm they are aware of this bug, that they can now (finally) reproduce it reliably, that they seem to understand the nature of the problem as well, and a fix is probably imminent.
Meanwhile, I also have more information about exactly what triggers the bug and how to avoid it, which may be helpful for many users.
The bug is triggered by copying certain strings of text. The problem strings are probably not very common. But some users, like myself, have been frequently triggering the bug because we habitually copy something that does do it. I have a spreadsheet that I work in constantly with dates formatted like this:
And most strings with a — followed by a : will break the clipboard history. By changing the date format in my spreadsheet (no dash), I have almost completely worked around this problem. It's basically a non-issue for me now.
There are other strings that can cause the problem, but they appear to be weird, like this one (previously reported in this thread):
Once triggered, the bug behaviour is consistent and comprehensible (just hard to work out without error logs). When active, the bug causes copied items not to be added to the clipboard until switching applications. Any subsequent copies before switching apps will be lost. It's was easy for me to verify this once I knew what I was looking for.
Finally, when clipboard history is broken, it can definitely be reset by toggling clipboard history off and back on. This became a viable workaround for me as soon as I eliminated the frequent triggers I was getting from that spreadsheet.