It looks like sh doesn't like the presence of parentheses in filenames and that's why it refuses to tab-autocomplete the files that have them. A year or two years ago, people said that sh couldn't tab-autocomplete commands and I don't recall people saying that sh couldn't tab-autocomplete certain types of filenames too.
It's more complicated. I just tried files abc, a(b, a)b and a*b, plus versions where the final "b" is replaced by "x". /bin/sh can autocomplete a(c, but not a)c or a*d. And it's not just the number of special characters, it can correctly autocomplete a(b(c versus a(b(x. There is a bug there, but it's not exactly clear to me what it is. To explore it systematically, you'd have to do a tree search of all strings that have one or more non-alphanumeric characters in them, and that includes special characters. Remember, the only character that does not exist in file names is "nul", all other 255 characters are possible. I have no idea how autocomplete would work with things like newline, backspace, or incomplete unicode sequences in the file name, and testing that would take hours.
Obnoxious side remark: I used to implement file systems for a living. One of the things I learned is that people who put strange special characters into their file names should all be shot. Or at least tickled until they laugh so hard they turn blue in the face. From an implementation point of view, it's trivial to allow people to create abominations like "a-with-accent, newline, i-without-dot-but-with-dot-accent, backspace, *" as a file name. Or the infamous directory whose first two file names are "-R" and "*" (both valid file names!). From a user point of view, these are catastrophically bad ideas.
Anyway, if you can update the bug with some observations of what patterns work and which don't, that may help the developer save some debugging time.