That looks OK, depending on your answer to this:
Plex doesn't write much except trancodes and possibly DVR if you use that. Do you use DVR in Plex?
Maybe one of your other dockers is to blame, such as a downloader since they write a lot of data.
There are basically 2 ways you can cause a container to write where it shouldn't.
If the host path in a mapping for any container isn't a disk or user share, then that host path is in RAM just like the rest of the Unraid OS, and if the application writes to the corresponding container path, it is writing into RAM.
More relevant for your issue, though. If an application writes to a path that doesn't correspond exactly to a mapped container path, then those writes go into the docker image. It is important to remember that Linux is case-sensitive and so case matters when specifying a path. And you must also make sure any application paths are absolute (or possibly relative to another absolute path already specified).
Specifying the wrong upper/lower case, or specifying a relative path, within the application settings is the most common reason for filling docker image. Note that this isn't really about the mappings, it is about the paths specified within the application itself not matching the mappings.