Answer
It's a known, acknowledged limitation, not a bug in your specific case — Update Analyzer's interface-impact analysis isn't usable at all when the comparison spans a pre-IFS-Cloud version to an IFS Cloud version (e.g. an Apps 10-to-Cloud upgrade path); IFS confirmed interface impact analysis isn't currently supported by the tool in general, regardless of the specific scenario. Layer impact analysis (checking whether your Cust-layer code overtakes something that changed) does still work and is detected correctly — it's specifically interface impact (argument/signature changes on a called procedure) that isn't reliably caught, so don't rely on Update Analyzer alone to catch this category of breaking change; manual/targeted review of customizations calling standard APIs is still needed around a service update.