Serial numbers are not showing up in the SNTC. I do see the serial numbers in the CSPC and can confirm this data is collected via the "ENTITY-MIB_entPhysical" dataset in the collection profile. Currently running version 188.8.131.52 and this is a fairly recent issue.
Could you please share customer company name, inventory and questioned device SNs that were not showing on portal? Please share couple of SN along with ip and hostname in private message. Please share screenshot of what you see under dataset for ENTITY-MIB_entPhysical.