Why can't I use group rollups in Jira Product Discovery?

[Tyler Gibson]

https://support.atlassian.com/jira-product-discovery/docs/use-filters-sorting-and-autosave/ 

This page says I can use group-by to then roll up numeric fields.

This doesn't seem to actually work?

I have tried custom formulas, weighted field, numeric fields, and rating fields - they all just display the rollup options as greyed out.

As seems to be the case more and more - none of this functionality is actually documented, nor is there any error or reason given in the UI for why rollups are disabled.

Help please?