While there are various reasons this could happen (Group limit, missing filters, out of sync DB, etc), one of the lesser known reasons is due to the Tag Management tool found under Governance > Tag Management.
For example, we can disable the "cleanup" tag:
This tag will still show in assets reporting:
But perspective searches/allocation will ignore this tag and therefore not allocate it:
You can re-enable the tag to allow allocation to continue on those assets if needed. This will take 24hours: