Entities disappear from boats (are removed, not invisible)

I have several cows in boats as part of an animal breeder. The boats are held motionless and are in vertical rows with 2 cows per boat. The boats are on iron bars, not in water. One column of boats will lose one cow each whenever I unload and load the chunk. The chunk is -16 4 31 in the seed MAP11. Most of the cow farm is in one chunk, but the three affected rows are in a different chunk.
I had this problem in 15W43c and updated to 15w44a to see if it was fixed. It was not.