Got it!!! When creating the XPM image of the squiggle to underline the spelling error Scintilla allocates a vector to hold it whose size is `width*height*4` and that doesn't allocate if the width or height is zero, so any index operation will likely crash.
Unfortunately the backtrace @hroncok posted has too many values optimised out/not printed to know where it gets that zero from, and why it only applies if the variant is applied to a character without variants and only on @hroncok's system