There’s a hidden list called TaxonomyHiddenList on each site collection in SharePoint that caches the terms used in that site collection. It takes time to update. The hidden list can be found on any site collection at this URL:
/sites/SiteCollectionName/Lists/TaxonomyHiddenList
In SharePoint on-prem there is an hourly (by default) Taxonomy Update Scheduler timer job that updates term changes on each site collection. In SPO we cannot run the timer job at our wish.
Reference: https://joannecklein.com/2018/11/26/changing-managed-metadata-values-in-sharepoint-online/
You can contact MS support team for more information.