I understand that it would be inconvinient to find flatter places to even put down what every you are carrying
(especially since animals can die anywhere and become liftables themselves)
but some things make no sense thus it has to be a bug. The only related report I could find is from 7 years ago!
It should not be possible to put down a container on a 200% slope or even a cliff!
There are even liftable items that cannot be build on ground that is too steep,
but once built nothing hinders you from putting it down on the same slope (example: Food Trough).
The slope I can maybe live with, but placing something into a cliff? This cannot be intended behaviour:
(Edit: formatting)