I'll say up front I don't know if this is a bug or not, but it's something I've noticed.
Because I sometimes run out of mudcrab chitin, I make a point of killing all mudcrabs I come across. Usually I get a chitin every 3-10 mudcrabs. But I've killed many, many mudcrabs in West Weald and they never drop chitin. It could be RNG, but I thought I'd post about it, just in case.