votdev I believe there is an issue with the creation and maintenance of a Raid 1 Mirror which I can replicate in a VM at present I am trying to assist the OP of this thread
I have an OMV5 VM which I use for testing various scenarios and I can actually replicate what the OP has done, I have 3 small virtual data drives in the VM.
Raid Management -> Create -> the dialog is fine, and all three drives are displayed, if I select Mirror with all three drives selected the Raid is created appears as clean but the size is correct for two drives, but devices used displays three, if I remove a drive from the array it appears as clean/degraded but shows two devices.
Likewise if I create a Mirror with two drives it appears OK, but I click grow or recover from the menu and select the third drive, this gets added, the size remains the same but the devices now show three.
Granted I have not created a file system but the Mirror option should only allow the user to select two drives only from the list displayed, a user should also not be able to grow a clean mirror.