SQL Server 2017 - Instance not applicable for update (CU31 GDR Build 14.0.3485.1)

I have a 2017 SQL Server, and I have tried applying the latest update to the instance (CU31 GDR, 14.0.3485.1) However, when running the update wizard, it shows that the instance is greyed out and "not applicable" for the update, even though it is running an older version. There are a few other SQL Servers that took the update just fine, while two others would not take it. The other two are at builds 14.0.3480.1 and 14.0.3475.1 (also CU31 GDR). The other servers/instances that took the 14.0.3485.1 builds were from the same previous builds. Anyone have any idea why this might be happening, and any suggestions on how to fix this so I may apply the updates?