Does it happen if you put a free-standing, accessible sink in the middle of the room with nothing near it or above it and with MOO turned off when placing it, then assign it as a kitchen sink? Does it happen on different lots in this save? Does it happen in a fresh save?
Can you share a screenshot (with all walls up around it) of the sink and any thought bubble they have when you tell them to wash a dish after designating all other sinks as bathroom and this one as kitchen (move the dishwasher to household inventory for this test)?