When you run a Discovery on a server that has multiple SSAS instances, only 1 SSAS instance is listed. This is visible by reviewing the upstream relationships in the CI. When you check the 'Audit History' of the SSAS CI the PID field would have updated multiple times, which indicates that the CI was being overwritten instead of separate ones being created for each SSAS instance.
Expected Behaviour: Separate CI's for SSAS instances are created.
Actual Behaviour: The same CI gets overwritten.
The most probable cause is that the "SSAS" pattern is outdated and was not written to handle multiple instances of SSAS.
We have modified the "SSAS" pattern and now it handles multiple SSAS instances.
You can see the modified pattern attached to this KB.
NOTE: Since changing the pattern will be classed as a customisation, you will need to revert the pattern back to Out of the Box version after future upgrades. You can find more information about the same in the below document Revert Customization