In UTF-8 mode, when Hunspell::suggest_auto is passed a word with capital letters, a segmentation fault happens in mkallsmall. It is indicated in hunspell.cxx that at least three experimental functions, suggest_auto, suggest_with_stems and morph_with_correction does not support UTF-8.
I attached a patch that modifies these functions to use mkallsmall2 instead of mkallsmall. I tested with en_US and hu_HU dictionaries. The behaviour now seems to be correct to me. (Although I don't really know what suggest_with_stems and morph_with_correction is supposed to do.)