This item has been identified as an outstanding known issue as of 14th April 2019. Please contact Support at firstname.lastname@example.org if you encounter this.
If a list has a parent list which contains between 2 and 999, or more than 1,999 items, the list index reset button will be greyed out and the list index can not be reset.
This issue will be seen even if all of the general prerequisites for a list index reset have been met, i.e.:
Go to Solution.
This issue was fixed in the May 18th, 2019 release.