The most inconsistent areas are those near water locks (both left and right). But it seems that it occurs anywhere (bathrooms, funghi plantation etc)
I was trying to trace this bug, so I've rebuild decontamination zones several times with no conclusion. It seems not related to problems with placing directional buildings (checkpoints, sinks) close to other buildings (like in checkopint, where it can't be placed near any other building, risen tiles included).
Since I was not able to define the bug, I am attaching my save file.
Steps to Reproduce
Play for some time, make several decontamination areas.
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.