According with Jira-Structure documentation for "Total Progress Calculation" at https://wiki.almworks.com/documentation/structure/9.1/cloud/progress-column-130883892.html you can specify how sub-issues progresses are aggregated into parent issue progress. But, when trying to group issues by Projects and using a numeric field to weight each "epic" issue, the calculation rolling up to the grouped line is not correct since the weighted progress uses not only the weights per each "epic" but also any other weight under sub-issues (histories, tasks and subtasks).
See the following calculation that is wrongly calculated for the grouped line (image #1 - Key CJC) showing 27%, against the workaround calculation (image #2 - Key CJC) showing 5%.
The problem we see is that when you group issues, the total progress calculation under an "issue progress based on field and weighted" the weights for the grouped line item considers all lines were there are weights included.
Has anybody solved this problem with. The workaround was to weight with round numbers the epics and to weight with decimals the sub-issues. (See image #2).
Image #1 - Wrongly calculated 
Image #2 - Work around
