This functionality is in technical preview and may be changed or removed in a future release. Elastic will apply best effort to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.
For version 8.5 and above we recommend downsampling over rollups as a way to reduce your storage costs for time series data.
Keeping historical data around for analysis is extremely useful but often avoided due to the financial cost of archiving massive amounts of data. Retention periods are thus driven by financial realities rather than by the usefulness of extensive historical data.
The Elastic Stack data rollup features provide a means to summarize and store historical data so that it can still be used for analysis, but at a fraction of the storage cost of raw data.