I have encountered a bug where the industrial skip isn't recognized as a good storage area for mechanoid corpses. To resolve this, I create a stockpile near my skip and say this is where the mechs should go... my colonists then race to grab those pesky mechanoid corpses and then... low and behold, drop off the bodies in the skip! But if I don't configure my stockpile (vanilla), the colonists appear to not understand the request.

I hope this was helpful in finding out where this disconnect is coming from.
https://gist.github.com/HugsLibRecordKeeper/ea1cd60eae1fafd91f6ea34f95da65c4