Is there a log or a way for me to confirm the reason why I can't grow a LUN (2016 Server) from 5 to 10TB? I suspect it's because it's more than 10x's original size, but I thought I started with larger LUN size.
I am not sure which Ontap logs would capture the 10x/max-lun warning/error, but in general you could try ::>event log show
But, as you are on Ontap 9.4, 10x resize limit applies.
In ONTAP 9.5, the 10x resize limit was removed, therefore normal resizing methods are available. In Clustered Data ONTAP 8.x and ONTAP 9 versions prior to 9.5, this capability is not present. Instead, a new volume and LUN should be created, and the data migrated to the new (appropriately sized) location. You can leverage copy offload on supported platforms to improve system utilization during migration to the new LUN.
Workaround prior to 9.5: Create the new LUNs at a maximum size of 16 TB with space reservations disabled. After the LUNs are created, they can be resized down (shrunk) to the desired size.