Geany displays each of them as a tiny square with the digits 0 0 0 0 inscribed.
That is a character with the value 0, which should never occur in the buffer because it will cause problems. It may even cause your crashes. Geany should not load files with that character in it, but maybe its encoded on disk as an overlong UTF8 so it loads but then wreaks its havoc.
Were you entering characters using some entry method, Scintilla (the project that makes the editing widget Geany uses) has had a lot of changes in that area recently and maybe it leaks 0 characters into the buffer. Unfortunately none of the Geany devs does much with texts beyond Latin so we probably have never even tried Scintilla's entry methods in Geany.
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