Hi Plunker, imo it seems a bit odd for the C string to be giving a buzz there, I'm just a bit curious how you tension your machine heads when re-stringing ? Reason - I've seen some people suggesting to tension the C string first then follow with others, and I wonder if you've tensioned the C string prior to tensioning other strings. What I usually do is start tensioning on one string and then go around the headstock tensioning each machine head (and string) in increments, so that the tension is applied evenly. I think of it as tensioning a Drum head, because my first instrument was drums (years ago now), when you tension a drum head you tension it evenly otherwise too much tension on one lug before others can create uneven tension on the head. likewise I've found although it's not very noticeable at first, the same thing can happen when tensioning machine heads (strings) and it can create uneven tension over the strings, headstock and neck. It's a suggestion worth considering but only you know how you have tensioned them at this stage. In this case, if it's applicable, you can loosen off the strings to release tension and simply tension them more evenly. If you have low G, and you've tensioned G and C strings first I suspect the odds of uneven tension the neck is even more so, and I would think that could be a cause.
(*While I'm mentioning drum heads , as some people have banjoleles, when tensioning a drum head tension the lugs in opposites to apply opposite tension evenly. In other words, start on one then start on it's opposite, then go to the one at 90 degrees from there and start on that one then it's opposite, then start on the ones between them and their opposite accordingly, this way you will get an even tension rather than an offest tension which you don't want. Just thought I'd throw that one in as you never know who's reading what, but this is irrelevant for this specific thread regarding the Uke)
I honestly have no idea if any of this will be helpful for you, but as said I think it's worth consideration.
It does seem an odd case , and I can understand why you think there's a possibility something is amiss, and there probably is, just finding out what it is could be a process of elimination, and loosening your strings and re-tensioning them evenly is an easy way to start that process, but whether or not you think that is applicable in your case I can't say.
If your Uke is one of those with a Truss rod, you could give a very slight tweak to release of the tension in the neck just a tiny bit, as it's possible there could be less tension in the new strings you are using. Sometimes the thicker strings tend to bounce a bit more than the thinner strings and it might just be a very slight tweak required to rectify that in this case.
And another thing, sometimes a neck will tension itself out better over a day or two after being strung up, I doubt I would wait on it though, and I'd simply check those other things, you'll figure it out.
imo all these things are rather unlikely, but all possible I suppose.