Hi, I’m a bit late to this conversation, but I still have some questions. I had the same problem as OP with Droid Sans Mono, and I’ll try out some other fonts.
What I don’t understand is why Droid Sans Mono would do this. The Unicode specification is that combining characters combine with the previous character, so why would they ever combine with the next character?
Based on the little testing I’ve done, when you have a greek letter, a combining character, then another character, the middle character combines with the last character, even if it’s also a greek letter. If the first character is an english letter then it will combine properly. I’ve also noticed that punctuation behaves similarly to greek letters in the above examples.