Grouping in Power Query|Power Query|Excel Forum|My Online Training Hub

A Word document isn't particularly helpful for testing purposes - any chance of an Excel workbook?

Here's one way. It creates a query for just the null date rows, groups by the Custom field and then converts the Explanation data into a comma separated text. That query is then merged with the original table - minus the null date rows - on the Custom field to append any new explanations.