It seems replacement pairs, at least when of different length, fail at the end of a word. E.g. "Fux" doesn't suggest "Fuchs" even with fsa.dict.speller.replacement-pairs=x chs. Adding two more characters at the end of the word makes it work (i.e. Fuxee suggests Fuchsee, both are fantasy words).

I've added a stand-alone example here in LanguageTool: https://github.com/languagetool-org/languagetool/blob/6d38df7eda58bfc6b796d91e35fc7debda57f7d5/languagetool-language-modules/de/src/test/java/org/languagetool/rules/de/GermanSpellerRuleTest.java#L369

Maybe this is related to this comment. If this is actually a bug and won't be fixed, it should be documented.