While I was writing this up, I believe I figured out the cause of my issue, but alas, I have no clue how to fix it.
I had to reboot my Hypervisor, and so I did. And then I blindly booted up all of my VMs. I completely forgot that all of my drives in OMV are encrypted, so they are not initially available upon boot. So after my other VMs booted up, autofs worked like a charm, and they dutifully tried writing to the NFS shares. It appears that they created data in the /exports/ folder on OMV, essentially breaking the NFS shares of OMV. I'll note that SMB is still working great, so this issue is specifically with NFS. But when I mount an NFS share, I see an empty folder, and the available space reported by the client-side file browser always shows as the amount of local storage on OMV, rather than the available space on the drive share. So what would be the procedure at this point to empty the /exports/ folder on OMV. I'm guessing I'd first disable NFS, and then run a command to wipe the folder contents (not sure where I'd perform this, or what the command would be), and then I could re-enable NFS on OMV and then turn autofs back on client-side.