I am having an odd issue that i was able to replicate using two different systems. This might be an issue with mergerfs, and not with OMV.
If i setup a disk pool using the policy "Most Free Space" with mergerfs, everything seems to work fine. I am able to write to the disk pool and read from it. The problem seems to be when I modify the contents in place... For example, If I rename a folder from A to A_renamed, both folders now exist. The original A folder has a few files, and then A_renamed has the rest (or visa versa)
2.) Before renaming folder B :
3.) I renamed folder 'B' to 'B_renamed'. Notice there are now both folders...
4.) Notice some files are still in 'B' and some files are in 'B_renamed'.
I wanted to use the policy "Most Free Space" because I want the data to be written across all the disks evenly until they are full. If I am doing something wrong, or need to edit a config file, or if I should use a different policy, please let me know.